It was a sagacious decision on his part.

Answer: Option C

Sagacious : having or showing keen mental discernment and good judgement; wise or shrewd.
Casual : relaxed and unconcerned.
Cunning : having or showing skill in achieving one's ends by deceit or evasion.
Foolish : lacking good sense or judgement; unwise.
False : not according with truth or fact; incorrect.

Antonym of Sagacious is Foolish