i received a private message regarding this thread.
satishchandra, i'm a little confused by the original post -- first, you say that (a) is the official answer, but then you ask how to eliminate (a).

i like choice (c) the best here. i will explain the differences between (a) and (c):

* MEANING
(a) contains "for", which carries the meaning of because. therefore, the meaning of sentence (a) is that jane and william are being invited to represent the school because of the work they have done this semester -- a reasonable meaning.
(c) contains a semicolon, which shows that the two clauses are related in meaning but doesn't exactly specify how. so, in this sentence, it's possible that jane and william are going to the convention for other reasons (such as a longer history of good work), but that the narrator still wants to emphasize the continuing quality of their most recent work. this is also a reasonable meaning.

* MODIFIER PLACEMENT
i don't like the placement of "this semester" in choice (a) -- it appears to modify the action/clause their creations have been outstanding. in other words, taken literally, choice (a) suggests that the creations are very good this semester -- but that, during other semesters, the same creations might not be so great. that's not very reasonable.
by contrast, (c) places "this semester" right next to their creations, specifying unambiguously that the creations were made this semester. that's better.

by the way, what is the source of the problem?

i've given about all the explanation i can muster, above.

I am not aware of the actual parent source

why would you do this to yourself?

DO NOT USE PROBLEMS FROM UNKNOWN SOURCES!

most third-party GMAT verbal problems are of extremely poor quality; if you go around solving problems from unknown sources, you will almost certainly (a) learn some things that are actually wrong, and (b) waste your time on a large number of things that are not actually tested on the GMAT.

you should stick with official problems; if you absolutely need more problems, then you should stay with problems provided by major, reputable companies.

[quote="apex231"In option A isn't it more obvious that Jane and William will represent school at convention because their creations were outstanding this season. [/quote]

in my post -- in fact, in exactly the part that you quoted -- i pointed out that either of those two interpretations would be reasonable. there's no point in debating between them; it's a badly written question (not surprisingly, given that it comes from no source in particular).

shekhar.kataria wrote:Is it correct to have the comma + for structure in the sentences such as above.

yes, that is fine.
"for" is a conjunction (not unlike "and" or "or"), so the construction is ok for the same reason that CLAUSE1 + , + and + CLAUSE2 is ok.