Final answer:
The starting friction must be c. equal to the force pulling the car downhill to prevent it from rolling. This ensures the car remains stationary, as friction counteracts the downhill gravitational force component.
Step-by-step explanation:
In order to prevent the car from rolling downhill while parked on a hill, the starting friction acting on the car must be c. equal to the force pulling the car downhill. This is necessary because if the force of friction was any less, the car would start to move downhill. If it was greater than needed, the car would remain stationary, and any additional friction would simply mean it's more securely parked, but it wouldn't change the scenario of just preventing motion.
The force pulling the car downhill is primarily due to gravity, which is acting on the mass of the car with a component along the slope of the hill. Friction works against this force to prevent the car from sliding. Therefore, in this static situation, the static friction force exactly balances the downhill gravitational force component, leading to no movement of the car.