Choose the sentence that is grammatically correct.

Principle: when two qualities of the same person or thing are compared, English prefers $more/less + adjective$ rather than the $-er$ comparative on both adjectives. Here, the sentence compares two qualities in the same person: bravery and strength. So the correct pattern is "more brave than strong." Option B is tempting because learners often try to make both adjectives comparative, but "braver than stronger" is not the accepted structure for comparing two qualities of one person. Option C is incorrect because it combines two comparative systems at once ("more braver"). Option D does not express the required comparison at all and uses an ill-formed structure. Therefore, option A is correct.