“Flippant” means ‘not showing a serious or respectful attitude‘.
Option A is the correct answer because “discourteous” means showing rudeness and a lack of consideration for other people, hence it is the correct answer.
Option B is incorrect because “ bitter” means having a sharp, pungent taste or smell; not sweet, which is not the best meaning of the given word.
Option C is incorrect because “humorous” means causing laughter and amusement; comic, which is incorrect because it is not the correct meaning of the given word.
Option D is incorrect because “casual” means relaxed or unconcerned, which is not the correct meaning of the given word.