DILIGENT :

A. Fool

B. Unhappy

C. Hardworking

D. Disappointment

Answer: Option C

Solution(By Examveda Team)

Fool :a person who acts unwisely or imprudently, a silly person.
Unhappy :not happy.
Disappointment :sadness or displeasure caused by the non-fulfillment of one's hopes or expectations.
Hardworking :tending to work with energy and commitment, diligent.
Diligent :having or showing care and conscientiousness in one's work or duties.

Synonym of Diligent is Hardworking.