The correct answer is D. I am interested in it.
The sentence “It interests me” is in the present tense. This means that the speaker is currently interested in the subject of the sentence. The other options are all in the past or future tense, which would not make sense in this context.
Option A, “I have been interested in it,” is in the past tense. This would mean that the speaker was interested in the subject of the sentence at some point in the past, but is no longer interested.
Option B, “I will be interested in it,” is in the future tense. This would mean that the speaker is not currently interested in the subject of the sentence, but will be interested at some point in the future.
Option C, “I was interested in it,” is also in the past tense. This would mean that the speaker was interested in the subject of the sentence at some point in the past, but is no longer interested.
Therefore, the correct answer is D. I am interested in it.