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