He was annoyed at his flippant remark.

Answer: Option A

Discourteous : showing rudeness and a lack of consideration for other people.
Bitter : having a sharp, pungent taste or smell; not sweet.
Humorous : causing laughter and amusement; comic.
Casual : relaxed and unconcerned.
Flippant : not showing a serious or respectful attitude.

Synonym of Flippant is Discourteous