He is a man of mellow temper.

Answer: Option B

Mellow : (of a person's character) tempered by maturity or experience.
Excitable : responding too readily to something new or stimulating; easily excited.
Hot : filled with passionate excitement, anger, or other strong emotion.
Irrational : not logical or reasonable.
Fickle : changing frequently, especially as regards one's loyalties or affections.

Antonym of Mellow is Hot