You are ___ than he.

This question tests knowledge of comparative adjectives. When comparing two things, the comparative form of an adjective is used.

• Cleverer is the correct answer. For most one-syllable and some two-syllable adjectives like 'clever', the comparative form is created by adding '-er'.
• Clever (B) is the positive form of the adjective, used when not making a comparison.
• More clever (C) is an alternative comparative form, sometimes used, but 'cleverer' is generally considered more standard and grammatically preferred for 'clever'.
• Most clever (D) is the superlative form, used when comparing three or more items, not just two.

Therefore, 'cleverer' is the most appropriate comparative form in this context.