The correct answer is: B. to solving
The infinitive “to solve” is used after the word “too” to express an inability or impossibility. In this case, the speaker is saying that the person is too dull to be able to solve the problem.
The other options are incorrect because they do not express an inability or impossibility. Option A, “solving,” is the present participle of the verb “to solve.” The present participle is used to form the progressive tenses, which express actions that are happening or continuing at the time of speaking. In this case, the speaker is not saying that the person is currently solving the problem; they are saying that the person is unable to solve it.
Option C, “to solve,” is the infinitive of the verb “to solve.” The infinitive is used after certain verbs, such as “want” and “need.” In this case, the speaker is not saying that the person wants or needs to solve the problem; they are saying that the person is unable to solve it.
Option D, “solves,” is the third person singular present tense of the verb “to solve.” The third person singular present tense is used to express actions that are happening or continuing at the time of speaking. In this case, the speaker is not saying that the person is currently solving the problem; they are saying that the person is unable to solve it.