The correct option is B spent / bought
The tense of a sentence denotes a time, whether it is the time now (present), time then (past) or the time to come (future). The verb of a sentence has most to do with the tense. In the given sentence, we need to fill in a word that determines the correct tense. The given sentence is in the past, we know that as it's mentioned "yesterday", and the second sentence is in relation to the first hence both are in the same tense. 'Spent/bought' are both in the simple past tense, which refers to the action(s) that were completed in the past. The speaker completed the action of spending money and buying a watch in the past itself, hence option B is correct. 'Spend/buy' are both in the present tense, whereas the sentence took place in the past, hence option A is incorrect. In option C, 'spent' is correct, but 'buy' is incorrect, as it's in the present tense, hence option C is incorrect. In option D, 'bought' is correct, but 'was spent' isn't, it's in the passive voice, whereas the given sentences are in the active voice. 'Was spent' in the passive form goes like this 'A lot of money was spent yesterday', not being the case here, hence option D is incorrect.