The correct option is A is / 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. The sentence highlights purpose or intention of an action. 'Is' is used with third person singular noun. Thus, options B, C and D are wrong according to the rule. The correct option is A since 'to' is in front of the verb 'buy'.