Answer: A) My friend or her brother was driving the car.
Explanation: Subject-verb agreement means the subject and verb must have the same number (both singular or both plural). When two or more singular nouns or pronouns are connected by "or" or "nor", we need to use a singular verb. In the given sentences the subject is "my friend or her brother" this are two singular nouns connected by "or" so the verb must be in singular form, in this case "was."