Answer: D) Subject.
Step-by-step explanation: In the given sentence we can see that we have two clauses (one is a conditional clause, because it has "if"), and they are separated with a comma. When we have two clauses, it is necessary that the subject (the person or group that performs the action of the verb) is clear in each one of the clauses. In the sentence "If it weren’t for my determination to finish the job, would be driving to school in a rusty old pick-up truck" it isn't clear who would be driving to school, so it is missing a subject.