The correct option is B dull
The word 'intelligent' refers to being brilliant and sharp.
Option A, simple refers to something easily understood or done.
Option B, wise refers to having experience, knowledge and good judgement.
Option C, weak refers to having little physical strength or energy.
Option D, dull refers to being slow to understand.
Thus, according to the meaning, the word 'dull' is the opposite of the word 'intelligent'.