STUPID :

Answer: Option B

Rude :offensively impolite or ill-mannered.
Illiterate :unable to read or write.
Obstinate :stubbornly refusing to change one's opinion or chosen course of action, despite attempts to persuade one to do so.
Dull :slow to understand; stupid.
Stupid :lacking intelligence or common sense.

Synonym of Stupid is Dull.