Which is the comparative form of the adverb high?

a.
most high

b.
highest

c.
more higher

d.
higher

The answer is:  [D]:  "higher" .

Note: 

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.