I did not do well on the writing exam,for the topic to write on was unfamiliar_________me.

句子中的"for the topic to write on was unfamiliar_________me"表示写作的主题对我来说不熟悉，其中"be unfamiliar to sb."是固定短语，表示对某人来说不熟悉的意思，因此正确的介词应该是to，答案为A。