The correct answer is: B. Dull.
Stupid is an adjective that means “lacking intelligence or common sense.” Dull is an adjective that means “not interesting or exciting.” Illiterate is an adjective that means “unable to read or write.” Obstinate is an adjective that means “stubbornly refusing to change one’s opinion or course of action.”
A person who is stupid may not be able to understand simple concepts or may make careless mistakes. A person who is dull may not be interested in learning new things or may not be able to hold a conversation. A person who is illiterate may not be able to read or write basic words or sentences. A person who is obstinate may not be willing to listen to others or may not be able to compromise.
In the context of the question, the word “stupid” is most likely being used to describe someone who is not intelligent or who does not have common sense. This would make the correct answer “dull.”