He was reasonable enough to behave according to the circumstances.

Answer: Option C

Peaceful : free from disturbance; tranquil.
Attentive : paying close attention to something.
Sensible : (of a person) possessing or displaying prudence.
Determined : having made a firm decision and being resolved not to change it.
Reasonable : having sound judgement; fair and sensible.

Synonym of Reasonable is Sensible