The correct option is A careless
The word 'vigilant' refers to keeping careful watch for possible danger and difficulties.
Option A, innocent refers to not being guilty of a crime or offence.
Option B, ignorant refers to being discourteous and rude.
Option C, irresponsible refers to not showing a proper sense of responsibility.
Option D, careless refers to being not concerned or worried about.
Thus, according to the meaning, the word 'careless' is the opposite of the word 'vigilant'.