The words "would best explain" indicate that this is an Explain question. Expect there to be a paradox or apparent contradiction involving last year's decrease in production. The answer will explain how both parts of the apparent contradiction can be true.
Untangle the stimulus in an Explain question by paraphrasing the two parts of the apparent contradiction. Here, pomegranate production, which had been unnaturally low due to manual pollination, increased greatly when the beetle was introduced five years ago. But then it decreased sharply last year.
It's difficult to predict an exact answer for most Explain questions. Just keep in mind that the correct answer will explain why pomegranate production suddenly decreased sharply last year despite the presence of the helpful beetle.
(C) explains why production suddenly dropped and is therefore correct. The introduction of the beetle caused rapid increase in production over the past five years, which ended up depriving the trees of nutrients needed for the development of fruit-producing flowers. The result is that the trees can't keep up their fruit production anymore.
(A) just describes how production affected prices, but it doesn't explain why production suddenly dropped.
(B) makes an irrelevant comparison between non-native trees and native trees. The stimulus is only concerned with one tree and its sudden drop in production.
(D) actually deepens the mystery. Had the snout beetle population dropped, it might have been understandable that pomegranate production would drop. But the snout beetle population has remained constant!
(E) pertains to an irrelevant time period - more than five years ago. This has no bearing on why production suddenly dropped in the last year.