Pronoun-antecedent agreement
12a. When a driver wants to buy a new vehicle, he or she usually test drives them.
12b. When a driver wants to buy a new vehicle, he or she usually test drives it.
My answer is: 12b
12a. When a driver wants to buy a new vehicle, he or she usually test drives them.
12b. When a driver wants to buy a new vehicle, he or she usually test drives it.
My answer is: 12b