He is too dull . . . . . . . the problem.

The correct answer is: B. to solve

The sentence is in the passive voice, so the verb “solves” should be changed to the infinitive form “to solve.” The infinitive form is used after the verb “too” to express an inability or lack of ability.

The other options are incorrect because they are not in the infinitive form. Option A, “solves,” is the simple present tense form of the verb “solve.” Option C, “solving,” is the present participle form of the verb “solve.” Option D, “to solving,” is not a grammatically correct form of the verb “solve.”

Here are some examples of how the infinitive form is used after the verb “too”:

• He is too young to drive.
• She is too tired to work.
• They are too old to play video games.