An adverb is a word or phrase that qualifies or changes an adjective, verb or another adverb, to express a relation of a situation, cause etc.
The correct answer is C)Adverb as it describes how much 'ripe' the fruit is.
Options A, B , and D are wrong as 'almost' is not a noun, not used in the given sentence to name the attribute of the noun (fruit), and it is not used in the place of any noun respectively.