this problem is terrible.
what is the source of this problem? hopefully the original poster will know the source, so that we can stay away from that source in the future!

i can tell that (c) is intended to be the correct answer, but there are still a couple of glaring mistakes.

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first, the problem is not formatted correctly.
on official problems, the underline ALWAYS starts at the first point where there is any difference between/among the answer choices, and ALWAYS ends at the last point where there is such a difference. no official problem would contain the same words at the end of all the choices ("relaxed in the dressing room with the impresario"), as this one does.

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second, "since" doesn't make sense; there are two meanings of the word "since", neither of which works here.

1) "since" can mean "from the date mentioned"; this usage of "since" must modify a clause whose verb is in the present perfect or past perfect.
ex: since thomas retired in 1971, no coach has achieved a winning record at the school. (up until now)
ex: the boy was sad because he had not seen his father since the previous month. (up until the time of narration, which is in the past)
--> this usage doesn't work here, because (a) it doesn't really make sense in context, and (b) the verb in question ("relaxed") is not in one of the perfect tenses.

2) "since" can suggest a cause-and-effect relationship.
this doesn't make a lot of sense in the present context, especially given the fact that the original, although wrong, presents the two events as a simple time sequence.

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third, the passive construction "has been finished" is not idiomatically correct here. if you write "X has been finished", then X has to be some sort of work of art, or construction job, or other such job/project whose completion has a physical manifestation. i.e., idiomatically, "X has been finished" implies that people can now come look at X.
for instance:
the mural has been finished --> this is idiomatically correct.
the party has been finished --> this is not idiomatically correct.

in this sentence, it would be more appropriate to write "had finished", or, better, "had ended".

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one way to rewrite the correct answer so that these issues are rectified:
once the concert had finished and the audience had filtered out, the maestro relaxed in the dressing room with the impresario.

these issues are probably a little bit too subtle be required of gmat students, but i'm pointing them out as evidence of the extremely substandard nature of whatever collection this problem is taken from.
in fact, the two mistakes above (the ones involving "since" and the passive) are errors that would be extremely uncharacteristic of a native speaker of english, leading to the conclusion that this problem (as well as whatever other problems came along with it) was probably written by someone whose first language is not english.
while that's admirable -- i would certainly not have the courage to write sentence correction problems in a foreign language -- it's important that the problems be authored by native speakers of english, so that you can become accustomed to the proper usage and style of the language.

paes wrote:Thanks Ron.

Source of the problem is gmat800.
Really this is a very bad questions.

You wrote all the problems that C have.

Similarly, can you write the problems that B have.

The only problem I see with B is pronoun ambiguity -> 'he'.

grammar error:

* "the audience had been filtered out" appears in the passive voice. if this wording is taken literally, it suggests that the audience was literally filtered out of the atmosphere (in the same way that, say, impurities would be filtered out of the air by an air purifier).
--> in this case, "filtered out" should appear in the active voice. (in this case, "filter" is used in a sense that means, roughly, "to move slowly, in a relatively orderly fashion").

meaning errors:

* as discussed above, it doesn't really make sense to use "because" -- this is not truly a matter of cause and effect. instead, it makes a lot more sense to write the sentence as a time sequence, as i did in my example above.

* the fact that "because" is used twice indicates that these are two separate, unrelated reasons why the maestro is relaxing in the dressing room. this also doesn't make sense, since the time sequence is really one reason, not two.

ex:
[b]since it was getting late and the sun was starting to set,[/b] we decided to go inside.
--> here, "since" is only used one time: "it was getting late and the sun was starting to set" is not really two separate reasons.

ex:
because we have to ship your order tomorrow and because you ordered custom designs, we are going to add substantial surcharges to the price of your order.
--> these are two entirely separate reasons for the increased price, so it makes more sense to use "because" twice.

in all honesty, this distinction is probably way too high-level to play a role on the gmat, but it's worth mentioning since it is, after all, one of the things that is not quite optimal about choice (b).

paes wrote:Thanks Ron.

Source of the problem is gmat800.
Really this is a very bad questions.

You wrote all the problems that C have.

Similarly, can you write the problems that B have.

The only problem I see with B is pronoun ambiguity -> 'he'.

oh, and, two more things:

* first, there is actually no ambiguity in choice (b): "maestro" is the only singular person that precedes the pronoun. to what else were you thinking this pronoun could refer?

* it also doesn't make sense to say that the maestro "finished the concert" -- this would imply that the concert was something that he gave all by himself, so that he could finish it all by himself. for instance, it's possible for a speaker to "finish his speech" -- because the speaker is the only one actually giving the speech.
for the same reason, a club promoter can't "finish a party". the promoter can host the party, but the correct usage is just to state that the party "finishes" (in active voice, no object).

paes wrote:Thanks Ron, for such a detailed explanation.

got some new concepts for the usage of 'because', two times.

glad it helped.

admittedly, some of that stuff is pretty advanced/subtle, but i want to do what i can.