Cattle egrets that stand on the backs of bovines pick off parasitic
bugs like ticks, fleas and flies while
egrets on the ground try to catch
grasshoppers or other insects
disturbed by the movement of the
cattle.

the answer is b because, investigating whether another species of moth also drinks tears might be interesting, but it would not directly explain why this species of moth does.

to explain why this species of moth drinks tears, the scientist could ask: "what nutrients that moths need are in elephant tears? "

the scientist could experiment with moth repellent or oter ways to keep the moths away, but that would not answer his original question.