The correct answer is: A. His whereabouts are not known to me.
The other options are incorrect because they do not accurately reflect the meaning of the sentence.
B. No one knows his whereabouts. This is not necessarily true. It is possible that someone else knows his whereabouts, but the speaker does not.
C. His whereabouts is not known by me. This is a grammatically incorrect sentence. The correct way to say this would be “His whereabouts are not known to me.”
D. His whereabouts had not been known to me. This is also a grammatically incorrect sentence. The correct way to say this would be “I had not known his whereabouts.”
The sentence “I don’t know his whereabouts” means that the speaker does not know where the person is. This could be because the person has not told the speaker where they are going, or because the speaker has not been able to find out where the person is.