The correct option is A as clever as
The question states that the sentence needs to be of the positive degree. A sentence is of the positive degree of comparison when the nouns in comparison have equal qualities. In the sentence, the boy is as clever as the other boys, "as clever as" is of the positive degree hence option B is correct. Option A is incorrect because the word alone shows no degree of comparison. Option C (cleverer) is incorrect because it is in the comparative degree. Option D (cleverest) is incorrect because it is in the superlative degree of comparison.