Concept:For two things, use either with a negative verb and neither with a positive verb.Solution:The verb is already negative: "have not bought". So we need either: "Ali's parents have not bought either of the two recommended books.""Neither" would make a double negative (not ... neither). "None" is also negative and is used for more than two. "Any" is used for more than two, not with "of the two".Answer:Option D (either)