The correct answer is (a) does he.
Question Tag Rules:
1. If the main sentence is Negative, the tag is Positive.
2. The tag pronoun must match the subject of the main clause.
3. The auxiliary verb in the tag matches the main verb tense.
Analysis:
— "Man hardly believes..." — "hardly" is a negative adverb (semi-negative), making the sentence effectively negative.
— Subject: "Man" → pronoun: "he"
— Main verb: "believes" (Present Simple) → auxiliary: "does"
— Negative sentence → Positive tag → "does he"
✓ Man hardly believes a liar, does he?
✗ "doesn't he" — would be used if the sentence were positive.
✗ "are they / aren't they" — wrong pronoun (subject is "Man" = "he").
Source: A Practical English Grammar; Cambridge English Grammar in Use.