The correct option is A to buy
The infinitive verb is a verb with the word 'to' in front of it. This verb is always just a verb. There is no - ed, no - ing, no - s at the end of the verb. Some verbs can be followed by question words + to-infinitive. 'What' is a question word here which should be followed by to-infinitive. Thus, options B, C and D are wrong according to the rule. The correct option is A according to the above rule.