In the air brake, air pressure is supplied by
A) Engine manifold
B) A compressor
C) The diaphragm valve
D) Pump

Hint: Air brakes work using compressed air instead of hydraulic fluid to stop the rotation of tires. Air brakes can be either drum brakes or disc brakes, or a combination of both.

Complete step by step answer:
In an air brake system, the air is pressurized by an engine-mounted compressor. Air brakes are a type of friction brake for vehicles in which compressed air pressing on a piston is used to apply the pressure to the brake pad which applies force on the tire that is needed to stop the vehicle. They are used in heavy vehicles like trucks busses, trailers, etc.
The air compressor then pumps the air into the air storage tanks, which store the compressed air until it’s needed. The parking brake engages by spring force in the parking brake portion of the spring brake chamber when the brakes are pressed and consequently the air pressure is released.
Hence it is a compressor that compresses the air to a high pressure required for the working of the system.

Hence option (B) is the correct choice.

Note: Air brakes are used in heavy commercial vehicles due to their reliability. The supply of air is unlimited, so the brake system can never run out of its operating fluid, as hydraulic brakes in normal vehicles can due to leakages. However, they are also costly to use as the compressed air has to be stored safely and pressure has to be maintained so that the air remains compressed and can be used for breaking.