Answer: How did Anne's diary help her cope?
Step-by-step explanation:
"How many years did Anne write in her diary?" will lead to a quick and succinct answer: "X years." This doesn't leave any room for conversation
"Is a diary considered an autobiography?" Will not lead to a conversation about Anne Frank's diary, rather a conversation about diaries in general.
"Do you have a diary?" Is, once again, not relevant to a conversation about Anne Frank's diary. Rather, it would lead to a discussion about your diary.
"How did Anne's dairy help her cope" is the best answer, because it could lead to a multitude of different topics all surrounding Anne Frank's diary and the ways in which she used it.
Hope this helps! :D