The correct option is A Bought
The passage refers to an action already completed. This is in simple past tense. Thus, verb required should be in its past participle form.
"Bought" in option A is correct as this is in the participle form of the verb. This is in tense agreement with the context.
"Buy" in option B is incorrect as this is the base form of the verb, whereas we need to use the past participle form of the verb.
"Buying" in option C is use din the continuous tense.
"Will buy" is used to indicate the future.
Thus, option A is the correct answer.