Though her collection of antique typewriters was quite extensive, Zoe had found that she had to spend very little time maintaining it.There are two sequence of events:
1st - She spent very little time maintaining
2nd - Zoe found
Thus 1st event should be in Past perfect and 2nd should be in simple past.
Zoe had found that she had to spend very little time maintaining it
Incorrect: 2nd event should be in simple past as explained above.Zoe found that she had to spend very little time maintaining them
Incorrect. The tense sequence is correct however, "them" should be singular since it is referring to a collection (collective noun)Zoe has found that she has had to spend very little time maintaining it
Incorrect. Same as A.Zoe had found that she has to spend very little time maintaining them
Same as A & BZoe found that she had to spend very little time maintaining it
Correct.Answer: E