Final answer:
A dual air brake system comprises two separate air brake systems controlled by a single set of brake controls, offering redundancy for safety.
Step-by-step explanation:
A dual air brake system refers to option a) 2 systems which use a single set of brake controls. This system is commonly found in larger vehicles, such as buses and trucks. It includes two separate air brake systems that are controlled by a single set of brake controls. The primary purpose of a dual air brake system is to provide a fail-safe in case one system fails; the other can still function and bring the vehicle to a stop. Each system typically operates one set of wheels—front and rear—or each system operates on both sets but is controlled independently.