this is not a gmat-like question. the answer is, at best, ambiguous, and the differences between the choices are way too subtle.

the first issue is that there are two possible interpretations that both make sense: (1) the firms transact deals and build databases through the panel of brokers, or (2) the firms transact deals and build databases through the brokers themselves. #1 corresponds to choices (c) and (d), and #2 corresponds to choices (a) and (b).
both of these are valid interpretations.

between choices (a) and (b), both are reasonable. this sentence makes sentence with either an essential modifier (the one lacking a comma, used rhetorically for something that's necessary to the meaning of the sentence) or a nonessential modifier (the one with a comma, used rhetorically for something that's descriptive but unnecessary). the meanings are slightly different - (a) implies that there might be other panels of brokers, while (b) implies that this is the only panel of brokers - but they both make sense.

choice (d) is wrong, because "through which" refers to "brokers". (it must refer to the word immediately preceding the comma.) that doesn't work, because "which" can't be used to refer to people.

choice (c) also makes sense, because the nonessential modifier ("through which they...") is allowed to refer to the whole noun phrase "panel of brokers". (see problem #50 in the og11 diagnostic test for evidence of this.)

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real gmat problems don't rely solely on distinctions that are this subtle, although they may bundle subtle distinctions with less subtle distinctions.

by the way, what's the source of this question? (whatever it is, it should now go on your list of dubious sources.)

and where's choice (e)?