The gatekeeper ought to be more vigilant these days.

Answer: Option C

Vigilant : keeping careful watch for possible danger or difficulties.
Irresponsible : (of a person, attitude, or action) not showing a proper sense of responsibility.
Ignorant : lacking knowledge or awareness in general; uneducated or unsophisticated.
Careless : not giving sufficient attention or thought to avoiding harm or errors.
Innocent : not guilty of a crime or offence.

Antonym of Vigilant is Careless