The pesticide Azocide, introduced to central valley farms

I go with C on this one.

The pesticide Azocide, introduced to central valley farms three
summers ago, has proven ineffective due to other pesticides already in wide use, whose chemical compositions have been neutralizing its
desired effect.

I vouch for "B", rationale goes like this:

For "c", whose seems ambiguous pronoun reference in the same ways, "E" also has "which"

Whereas, "B" is concise having clear pronoun reference and clearly saying "chemical compositions of other pesticides are in wide use "

Vivek, please reveal official answer..

Dharmin

The correct answer to this question is (C).

The original sentence (A) is faulty in two respects. First, it improperly uses "because" instead of "because of." Second, the construction leaves it unclear as to whether the modifying phrase "already in wide use" refers to "other pesticides" or to "chemical compositions."

(B) misuses the phrase "because of." The author should follow this prepositional phrase with a modifying clause by inserting a comma followed by "which" after "in wide use" ("in wide use, which").

(C) corrects the misuse of "because" by replacing it with "due to."

(D) is redundant in its use of both "since" and "due to." This redundancy results, in turn, in an awkward construction.

(E) misplaces the adverbial clause "already in use," which is intended to refer not to "chemical compositions" but rather to "other pesticides."

Vivek.

Awesome vivek
Thanks indeed pal !
I should really acknowledge your such a meticulous reply for "B". Chemical compositions of the pesticides is a noun phrase requires comma and relative pronoun like "which" followed to it.

Dharmin

OG clearly says "due to" is synonymous with "attributable to".