The correct answer is weaken.
The correct answer is (A) To weaken; lessen in quality or strength; diminish in value or excellence, etc. To make worse or cause to become worse.
Impair comes from Middle English empairen, Middle French empeirer (to make worse).
Examples of use:
- Inadequate warmth could impair our health.
- Their failure to agree on the contract term impaired negotiations on other issues.