The given sentence needs to be filled with an adverb of frequency. An adverb of frequency tells us how often the action takes place, i.e. the rate of occurrence. Thus ‘twice a week’ is the apt answer. Hence option C is correct. Whereas ‘tomorrow’ is an adverb of time, which tells us when the action happened, i.e. the time. So option A is incorrect. Similarly ‘nearby’ cannot be used, because it is an adverb of place. It indicates where the action happens or where someone is going. Hence option B is incorrect. ‘Slowly’ is an adverb of manner. It explains the way people do things and is not used to indicate the frequency of the action. Therefore option D is incorrect.