The answer is:Â [D]:Â "higher" .
Note:Â
Choice: [A]: "more high"; — is grammatically incorrect at all costs.
Choice: [B]: "highest" ; — is the "superlative" — NOT the "comparative".
Choice: [C]: "more higher"; — is grammatically incorrect at all costs.
Choice: [D]: "higher" — is the comparative form and is grammatically correct.