Which example illustrates an Ad Hominem fallacy?

An Ad Hominem fallacy occurs when an argument attacks a person's character or circumstances rather than addressing the substance of their argument. Option A serves as a clear illustration of this fallacy because it involves a politician rejecting an opponent's argument solely based on that individual's past moral failures, rather than engaging with the merits or logical consistency of the argument itself. This approach diverts attention from the actual issue at hand and instead seeks to invalidate the argument by undermining the opponent’s credibility based on unrelated personal history.

The other choices do not represent Ad Hominem fallacies. The teacher's argument against a student's point of view is focused on the discussion itself; the attorney's explanation is based on facts, likely discussing a legal matter without resorting to personal attacks; and the doctor's medical advice, although potentially lacking context, does not target the character or personal traits of the individual involved.