Why distilled water used for the preparation of lassaigne's extract?

This is because tap water contains chloride ions which will give a precipitate of AgCl with AgNO3 solution even if the organic compound does not contain chlorine.

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.