Professors are generally serious about what they say.

A. jolly

B. thoughtful

C. smug

D. insincere

Answer: Option D

Solution(By Examveda Team)

Serious : demanding or characterized by careful consideration or application.
Jolly : happy and cheerful.
Thoughtful : absorbed in or involving thought.
Smug : having or showing an excessive pride in oneself or one's achievements.
Insincere : not expressing genuine feelings. Antonym of Serious is Insincere