Since these wasps eat nothing but stem borers, importing them into North America will keep crop damage from stem borers under control without endangering other North American insect species.

This is a Statistical type assumption. The author is assuming that similar to what happened in other continents, the same would happen in North America.
Whenever we get Assumption question, we should always use Negation Technique to eliminate the answer choices.

(A) Corn is the principal food of stem borers that live on continents other than North America. (What if Corn is not staple food? Does it affect our conclusion? No. So this is out)
(B) The wasps are capable of surviving in North America long enough to eat significant numbers of stem borers.(What if they are not capable? Does it affect our conclusion? Yes, by implying that similar to what happened in other continents, the same is not happening in North America. So keep this)
(C) No wasp in North America is closely related to the species of wasp that eats stem borers.( What if they are closely related? Does it affect our conclusion? No. So this is out)
(D) On continent other than North America, the wasps control stem borers more effectively than does any other pest control measure. (Does it matter how efficient wasp control? No. Out of scope)
(E) Corn crops on continents other than North America are not damaged by any insect pests other than stem borers.( What if other continents' crops are damaged? Does it affect our conclusion? Not at all. Out of scope)

Hence, B