A leaky exhaust system should be repaired because it:

Repairing a leaky exhaust system is particularly important because it can allow dangerous fumes, such as carbon monoxide, to enter the vehicle's cabin. This poses a significant safety risk to the occupants. Carbon monoxide is a colorless, odorless gas that can be extremely harmful or even fatal if inhaled in large amounts. Ensuring that the exhaust system is sealed properly helps to maintain a safe environment inside the vehicle and protects passengers from exposure to these toxic fumes.

While the other options address consequences of a leaky exhaust system, the direct impact on passenger health and safety underscores the critical need for repairs. Wasting gas and oil, causing the engine to run hot, and making it hard to start or stall might be valid concerns for vehicle performance, but they do not carry the same immediate danger to human life as the risk of harmful fumes entering the car.