An adverb is a word or phrase that qualifies or changes an adjective, verb or other adverb, to express a relation of situation, cause etc.
The correct answer is C)happily as the given word 'happy' ends with 'y' , so to make an adverb out of it, 'y' is replaced by 'ily'
Options A,B, and D are wrong as they are misspelled.