The correct options are
C Bought
D Had bought
Generally, we use the past perfect tense to indicate that two events occurred in the past in which one event was completed before the other. In this context, option C) had bought would be a possible answer. This would then imply that the person had bought the scooter the previous week before some other event. Options A) had brought and B) brought are incorrect because 'brought' is the past participle form of the verb 'bring', not 'buy'. Option D) bought is also another possible answer since 'bought' is the past participle form of the verb 'buy'.