Answer:
B.
Explanation:
A and C have awkward sentence structure because "when a car needs many serious repairs, it might be cheaper to buy a new car", it repeats the word car again making it redundant and in C, "When a car had needed many serious repairs, it might be cheaper to buy a new one", it has wrong use of past-tense.
Jack is a subject. She and he are subject pronouns. So the answer must be A.