Hey All,
This is a very bizarre question, and I'm afraid I have to disagree with everyone who's saying the answer is "obviously" A. I see nothing wrong with B (which makes this problematic as an official answer). Also, there are serious problems with GMAT Tiger's explanation, which many people have been describing as sensible.
He wrote:
A is correct. "That" in the sentence refers to "green fields", which is plural and so does "that" too. [What in the world does this mean? How can something "do 'that'"?
When "that" is plural, the verb that follows "that" should be also plural (surround), which is only in A and D. [THAT is a relative pronoun, and has no plurality. There is no such thing as a singular or plural "that".
"many of them" is better than "many of whom". -- [This is NOT true. "Them" is a subject pronoun and "whom" is an object pronoun. They have totally different uses. In fact, our ears prefer "them", so GMAT will often encourage you towards "whom".]
OKAY! Now that we've dealt with that bit of confusion, we can look at the question.
In good years, the patchwork of green fields that surround the San Joaquin Valley town bustles with farm workers, many of them in the area just for the season.
(A) surround the San Joaquin Valley town bustles with farm workers, many of them
PROBLEM: The writing of this sentence makes it impossible to work out what the subject of surrounds is. It could be fields (because relative pronouns like "that" typically modify whatever they touch), but it could also be patchwork (because "of green fields" is a modifier, just as we could say the King of Spades IS a good card). This sentence is extra confusing because "surround" and "bustle" are both verbs. The A folks in the room seem to want to argue that the "fields" surround the valley, but the "patchwork" bustles. But why? The same subject could just as easily do both.
(B) surrounds the San Joaquin Valley town bustles with farm workers, many of whom are
CORRECT: Whom is correct here, because we need the object of a preposition ("of"). I prefer this answer myself, but I see no actual reason to cross it off.
(C) surround the San Joaquin Valley town bustles with farm workers, many of who are
PROBLEM: You can't say "many of who", you need an object pronoun to be the object of the preposition "of".
(D) surround the San Joaquin Valley town bustle with farm workers, many of which
PROBLEM: This verb arrangement is odd, as is "many of which". We prefer who/whom for people.
(E) surrounds the San Joaquin Valley town bustles with farm workers, many are
PROBLEM: This starts a new sentence after the comma, which isn't allowed.
Hope that helps, and I'm definitely up for argument! : )
-tommy