The correct answer is: D. have seen
The verb “to see” can be used in the simple past tense (saw), the present perfect tense (have seen), or the past participle (seen). In this case, the correct tense to use is the present perfect tense, because the speaker is asking about a past event that has a bearing on the present. For example, if the speaker is asking about a trip you took to India, they would say, “Have you ever seen the Taj Mahal?” This is because the speaker is interested in whether you have seen the Taj Mahal in the past, and whether that experience has had any impact on you in the present.
The simple past tense (saw) would be used if the speaker was asking about a specific time when you saw the Taj Mahal. For example, if the speaker was asking about a trip you took to India last year, they would say, “When did you see the Taj Mahal?”
The past participle (seen) would be used if the speaker was describing a painting or photograph of the Taj Mahal. For example, if the speaker was showing you a picture of the Taj Mahal, they might say, “I’ve seen this before. It’s the Taj Mahal.”
I hope this explanation is helpful!