i'd go with (c), for at least two reasons:

* i'd give the nod to the idiomatic expression discriminate between X and Y over the alternative discriminate X from Y.

* i've also never seen a modifier stuck directly between a VERB and that verb's DIRECT OBJECT, as in (a) and (b); that is just ridiculously awkward. (c) fixes this problem because there's no direct object; the modifier is inserted before a prepositional phrase. that's common practice.
example of why the former is bad:
with a child's telescope you can see, though only with extreme difficulty, jupiter --> uh, no.
with a child's telescope you can see jupiter, though only with extreme difficulty --> better.


still, i'd hesitate to accept any decisions on idiomatic usage from any authority other than the official guide / gmatprep. is this an official problem?

Vignesh.4384 wrote:Hi Ron,


I just changes the sentence construction a bit.
Do you think the sentence constrctuin below is valid ?


After a few weeks’ experience, apprentice jewelers can usually begin to discriminate genuine diamonds from imitation diamonds, though not with absolute certainty.

Please advise

Regards,
Vignesh

that construction is valid according to several different english language authorities, but don't ever forget: the only authority whose opinion matters, for the purposes of gmat problems, is the gmat.
for sure, the modifier is placed correctly.

in any case, i would consider that construction somewhat archaic. in any case, it's certainly less common than discriminate between X and Y, and i would GUESS that the gmat would prefer the latter.
i will not make any definitive statements, though, until i see an official problem that decides the issue one way or the other.

reachac wrote:Hello Ron,

I have read somewhere that "discriminate X from Y" is the usage when we are try to distinguish two rather similar thing for example the original one from the imitated one.

And that "discriminate between X and Y" is the usage otherwise.

Can u explain the usage please. Also what would be the answer to the question in the original post?

i believe this issue has already been addressed as well as possible in the post above, but some additional comments follow.

--

beware of any sentence that starts with "i have read somewhere". remember the following two points:

(1) there are multiple authorities on english grammar and usage, no one of which is definitive or dominant, and these authorities often disagree on niceties of usage (such as this one).

(2) the conventions of usage adopted by the official gmat will sometimes be at odds with the conventions followed by other sources.

--

as i said above, many english language authorities accept "discriminate X from Y"; others classify it as archaic or awkward.
i don't know the gmat's official line on this issue, because i haven't seen the distinction drawn in an official problem.
however, i have a strong intuition that the gmat would prefer "discriminate between X and Y" to "discriminate X from Y".
i would love to see any evidence, or counter-evidence, from OFFICIAL SOURCES on this issue.