answer:
The correct answer is C: "haven't seen".
In the sentence, the verb "have seen" should be in the past tense to match the time frame mentioned in the question ("haven't seen each other for a long time"). Therefore, the correct form should be "hadn't seen" instead of "haven't seen".
Here is the corrected sentence:
"He wondered what his sister looked like because they hadn't seen each other for a long time."
In this corrected sentence, the verb "hadn't seen" reflects the past tense and maintains consistency with the time frame mentioned in the question.