6518. The inspector was a vigilant young man.

✅ The correct answer is option C.
intelligent : having or showing intelligence, especially of a high level.

ambitious : having or showing a strong desire and determination to succeed.

watchful : watching someone or something closely; alert and vigilant.

smart : (of a person) clean, tidy, and well dressed.

vigilant : keeping careful watch for possible danger or difficulties.

Synonym of vigilant is watchful