By warning him prior to action, he has landed himself in a precarious situation.

Answer: Option B

Precarious : not securely held or in position; dangerously likely to fall or collapse.
Carefree : free from anxiety or responsibility.
Safe : protected from or not exposed to danger or risk; not likely to be harmed or lost.
Easy : achieved without great effort; presenting few difficulties.
Cautious : (of a person) careful to avoid potential problems or dangers.

Antonym of Precarious is Safe