I think A is fine.

As far as i know...the use of "which" in GMAT always requires a "comma" before it .

If there are exceptions then please share.

I didnt pick C because it says "
the best junior employees compete for the most attractive "

I dont think it says anywhere in the original sentence that only the "Best Junior" employees compete...i took that as a change in the meaning.

there are lots of exceptions. which is always preceeded by a comma if it is starting a nonrestrictive clause. anyone that scores about 700 needs to know which is a pronoun.

of which
through which
under which

there are too many to list. you just have to pick apart sentences and ask why the sentence is structured the way it is.

I believe C is correct after zeroing on to A and C because of 'where'. Since Market place is a location, 'where' is more suitable than "which" or "in which" atleast in this sentence.
C is more precise and clear when in the later part of sentence when it says : "the best junior employees compete for the most attractive assignments". A also says the same thing but with more words..

Hence C.

wht's OA ?

I had picked C too, but OA is E

This question uses the incorrect pronoun "where" to refer to "informal talent marketplaces." "Where" is used to refer to physical locations, but the "talent marketplaces" are a metaphorical construct, not an actual geographical location. In this case, "in which" is the appropriate construction. The original answer choice also utilizes unnecessary turns of phrase in its use of “best employees junior to them,” as well as “assignments that they find most attractive,” both of which use excessive language and could be expressed more succinctly.

(A) This choice is incorrect as it repeats the original sentence.

(B) This choice correctly changes "where" to "in which" but incorrectly uses the conjunction “and” between the terms “strive” and “identify,” suggesting that the two terms denote two distinct activities, as opposed to the intended single activity of identifying the best junior employees. Also, “assignments that they find most attractive” are unnecessarily wordy.

(C) This answer repeats the original pronoun error ("where").

(D) This choice incorrectly uses the conjunction “and” between the terms “strive” and “identify,” suggesting that the two terms denote two distinct activities, as opposed to the intended single activity of identifying the best junior employees.

(E) This choice correctly changes "where" to "in which" and also rephrases the sentence more concisely ("best junior employees"; "most attractive assignments").