Several years ago, as a measure to reduce the population of gypsy moth

I am absolutely clueless on all answer choices for this question.

GMATNinja, nightblade354, VeritasKarishma,
can anyone please help here?

Thanks VeritasKarishma,
I was able to rule out B, C and D. However, even A and E looked distant to me.
Now, with your application it all makes sense.

Several years ago, as a measure to reduce the population of gypsy moths, which depend on oak leaves for food, entomologists introduced into many oak forests a species of fungus that is poisonous to gypsy moth caterpillars. Since then the population of both caterpillars and adult moths has significantly declined in those areas. Entomologists have concluded that the decline is attributable to the presence of the poisonous fungus.

Which one of the following, if true, most strongly supports the conclusion drawn by the entomologists?

(A) A strain of gypsy moth whose caterpillars are unaffected by the fungus has increased its share of the total gypsy moth population. - CORRECT. Non affected species increased it share of population either by increasing its population or keeping it population at certain levels prior to introduction of fungus does suggest that affected species declined because of that fungus.

(B) The fungus that was introduced to control the gypsy moth population is poisonous to few insect species other than the gypsy moth. - WRONG. Again what happens to other species is irrelevant.

(C) An increase in number of both gypsy moth caterpillars and gypsy moth adults followed a drop in the number of some of the species that prey on the moths. - WRONG. Irrelevant at best.

(D) In the past several years, air pollution and acid rain have been responsible for a substantial decline in oak tree populations. - WRONG. For me it is second best. It is so if and only if someone considered looking for answer of why the population decreased. Otherwise it is not suggesting whether fungus was responsible for such a decline. Instead it weakens the claim.

(E) The current decline in the gypsy moth population in forests where the fungus was introduced is no greater than a decline that occurred concurrently in other forests. - WRONG. Sure it might be true but was that fungus responsible for such a scenario? Not certain about about it.

There are to claims that one may look for in which only one is true.
1. Why population decreased?
2. Fungus was responsible for population decrease.

First, is wrong since it was given already in the passage and only if someone misunderstands the passage he/she may look for strengthening it.

Answer A.

(A) A strain of gypsy moth whose caterpillars are unaffected by the fungus has increased its share of the total gypsy moth population.
Total GM population constitutes of (GM affected by Fungus+ GM unaffected by Fungus). This option clearly states that the unaffected GM caterpillars has increased the population, implying that the other part type of GM died. If had not increased the GM population would have been decreased.
(B) The fungus that was introduced to control the gypsy moth population is poisonous to few insect species other than the gypsy moth.
Irrelevant
(C) An increase in number of both gypsy moth caterpillars and gypsy moth adults followed a drop in the number of some of the species that prey on the moths.
Incorrect. It means that the predators may have contributed to the low GM population.
(D) In the past several years, air pollution and acid rain have been responsible for a substantial decline in oak tree populations.
Incorrect. attributing to the availability of food
(E) The current decline in the gypsy moth population in forests where the fungus was introduced is no greater than a decline that occurred concurrently in other forests.
So, the fungus has not been specially effective

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