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