The adverb in the given sentence is ‘skilfully’. It is an adverb of manner that explains the way people do things. Therefore option A is correct. Whereas an adverb of time will tell you when the action happened, like ‘tomorrow’ or ‘next year’. An adverb of frequency tells us how often the action takes place, like ‘regularly’ or ‘twice’. While an adverb of degree explains the intensity of the action, like ‘extremely’ or ‘scarcely’. Since the given adverb explains how, i.e. the manner in which the person plays the saxophone, options B, C, and D are incorrect.