What is the correct superlative form of the adjective to complete the sentence?

The superlative form of an adjective is used to denote the highest degree of comparison among three or more entities. In the case of the adjective "high," the superlative form is "highest." This usage indicates that something is at the top of a comparative scale, meaning it surpasses all others in height.

When forming superlative adjectives in English, typically, one-syllable adjectives, such as "high," take the "-est" ending, resulting in "highest." This form effectively communicates that there is nothing higher than the subject being described.

The other options do not fulfill the requirements for a superlative form. "Higher" is a comparative form and is used when comparing two things; "high" is the base form and does not convey any comparison; "most high" is not a standard construction in English for this context, making "highest" the appropriate choice.