At Shiprock, New Mexico, a perennially powerful girls’ high school basketball team has become a path to college for some and a source of pride for a community where the household incomes of 49 percent of them are below the poverty level.

(A) where the household incomes of 49 percent of them are
(B) where they have 49 percent of the household incomes
(C) where 49 percent of the household incomes are
(D) which has 49 percent of the household incomes
(E) in which 49 percent of them have household incomes

Please remind me of when to use to Which VS Where. I am also not sure if I got this question correct, by luck or because my reasoning was correct.

I felt the problem with using "Which" in the above sentence is that "Which" refers directly back to "Community" and the sentence should refer back to Shiprock New Mexico. If so why does "Where" do that?

I elimanted D,E
I next elimanted A,B because of pronoun issues "They" and "Them" does not seem to have a clear antecedent
Leaving me with the answer Choice C.

Generally "where" is used for physical locations - otherwise, we have to use a variant such as "in which." Other variants include "at which" and "for which." Both "where" and "in which" refer to the community, which has already been established as a physical location (Shiprock New Mexico), so we can use "where." We could also use "in which" - this just means "in the community" which is also grammatically correct.

The key is that the only "in which" option given has a pronoun error, so I don't have a grammatically correct option that uses "in which." That leaves me with "where" by default.

A, B, and E all have pronoun errors. D would have to have a comma before the which and the verb "has" makes the sentence awkward. The community doesn't "have" household incomes. The individual members of the community do.

I didn't quite understand Stacey's point in which she mentions that which has already been established as a physical location (Shiprock New Mexico), so we can use "where." . Can someone explain this?

Is it mandatory for a (grammatically correct) construct with "which" to have a comma always infront of "which" ?

Thanks in advance.

stacey is saying that you can't use the word 'where' unless you've established that you're talking about a physical location. in this particular problem, you have indeed established such a thing, because shiprock, n.m., is definitely a physical location.

stacey is not saying that the word 'which' has been established as a physical location, in case you read her sentence that way.

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re: your other question
as far as we can tell from our rather extensive research, the gmat does not use 'which' without commas. if you have a modifier that is not preceded by a comma - i.e., an essential modifier - then you use 'that', not 'which'.