Final answer:
The correct answer is (1) Emergency, which refers to the third type of brake in modern air brake systems, along with service and parking brakes.
Step-by-step explanation:
Modern air brake systems in vehicles are comprised of three different systems: the service brakes, the parking brakes, and the emergency brakes. The option (1) Emergency is the correct answer to this question. When regular service brake systems fail, the emergency brake system activates to stop the vehicle. This is accomplished through a backup system that can either be spring-activated or have a separate air tank that can be utilized in case of a primary system failure. Heavy-duty vehicles, such as power brakes and those in bulldozers, may have a motorized pump that aids in the operation of the system, providing the necessary force to activate the brakes in an emergency situation.