The correct answer to this question is Infinitive. A verb is infinitive when the verb has the word "to" in front of it. The verb in this sentence is bought. It has the word "to" in front of it, which makes the verb buy infinitive. Another example would be in the sentence, "He went to the kitchen to cook for his family."
In this example, cook is the verb, and it has the word to in front of it, making the verb cook infinitive. An infinitive verb can serve many purposes. One purpose is to be a subject of the sentence. Another purpose is to follow certain verbs.