Answer:
4. Toni Morrison's novels aren't difficult for readers familiar with William Faulkner's work.
- William Faulkner and Toni Morrison are single nouns referring to her books. So, we need apostrophes after those.
5. He rebuilt his parents’ house after it was destroyed by a fire in 1991.
- When using the apostrophe with a plural noun in "parents," the "s" makes this word is no longer part of the original sentence.
Step-by-step explanation:
The details are in the above answers.