The inspector was a vigilant young man.

Answer: Option B

Smart : (of a person) clean, tidy, and well dressed.
Watchful : watching someone or something closely; alert and vigilant.
Ambitious : having or showing a strong desire and determination to succeed.
Intelligent : having or showing intelligence, especially of a high level.
Vigilant : keeping careful watch for possible danger or difficulties.

Synonym of Vigilant is Watchful