Which of these possibilities depends on exactly what the question is.
2) "Mr. Keanu is not on the committee" is the most likely answer (since the original question isn't given). It means that Mr. Keanu is not a member of the committee.
1) "not at the committee" isn't entirely proper grammar, but could be understood as meaning that Mr. Keanu is not at an ongoing committee meeting.
3) "not in the committee", like 1), isn't entirely proper grammar, but could be understood as meaning either that Mr. Keanu is not a member of the committee, or is not present at an ongoing meeting of the committee.
4) "not in committee" is grammatical, but has a very particular sense, and should be used only in very specific circumstances: Mr. Keanu is a member of the committee, but he is absent from an ongoing committee mneeting.