also, i want to note that the structure in choice (c), when used with a proper noun, is actually ALWAYS wrong -- in this sort of situation, the proper noun would actually have to be preceded by "the".

examples:
let's say that two people named Jimmy Smith both try out for an audition in a play. one of these people tries out on wednesday, and the other tries out on thursday; furthermore, the director only wants to see the second one for a callback.

the correct way to write the sentence is
the director wants to see the jimmy smith who tried out on thursday.
note that you use an essential modifier here, since you have to narrow the meaning of "jimmy smith" -- but the construction is wrong unless you also used the word "the" beforehand.

--

in the much more common situation in which there is only one person by a given name, you of course do not use the word "the", and you use nonessential modifiers (since you are not narrowing the possibilities for anything).

e.g., let's say that only one person named Tommy Wallach tried out, this time on friday:
the director wants to see Tommy Wallach, who tried out on friday.

--

also, i am 99.9999% sure that you will not really have to know any of this stuff, although it's useful in eliminating choice (c) in this (admittedly completely non-gmat-like) problem.