Sorry for those who read my previous answer. I had time to reread it and it seemed wrong to me and I did not want to mislead anyone. I will go with A. Final answer. First, ask yourself the question: What surround(s) the San Joaquin Valley? The "patchwork" or the "fields"? The "fields" it is and we can eliminate B and E.
C) many of who --> "whom", the object form is needed here. Ask yourself the question: Is many of "them" or many of "they" right? Them it is and you should pick "whom".
D) "whom" is prefered to "which" when refering to a person/group of person
My previous explanation about A not having a verb is wrong because "many of them in the area just for the season" is a noun phrase (no verb needed) which modifies "farm workers". I believe the usage of "whom" would have been better but it is also not grammatically wrong to use "them"