An adverb is a word or phrase that qualifies or changes an adjective, verb or another adverb, to express a relation of a situation, a cause etc.
The correct answer is option C)highly as it ends in 'ly' and is spelled correctly.
Options A,B, and D are wrong as 'higher' is a comparative adjective, 'highest' is a superlative adjective, and the word ii misspelled respectively.