Answer:
b. As part of the continuous form of a verb
Explanation:
The verb in the sentence "My father bought that building" can perform different functions but in this case, it can function as part of the continuous form of the verb because the verb "bought" is a past tense of the base verb "buy" but it can also act as a present continuous tense because the building is already the property of his father so it is a present continuous form of a verb.
