The adverb in the given sentence is ‘downstairs’. It is an adverb of place which explains where the action happens or where someone is going. Therefore option A is correct. Whereas an adverb of manner explains the way people do things, like ‘skilfully’ or ‘correctly’. 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 where the brothers were playing, i.e. the place, options B, C, and D are incorrect.