The correct answer is (C) To dishonor; to tarnish or soil; to detract from (the honor, luster of; etc.); to dim the brightness of.

Besmirch comes from Latin be (completely) and English smirch (to stain).

Examples of use:

• The players' misdeeds besmirched the team's good reputation.
• The candidate besmirched his rival's honor.