D is wrong for either...or....?
For b, past generations had preserved for us=books?
(D) is the correct answer choice. It basically paraphrases the argument in a slightly more general way since it refers to anything, not just books. The first sentence says we should preserve books. The second sentence tells us why, to benefit future generations. Instead of using the phrase 'future generations', (D) uses the word posterity, which means the same thing.