2a] how does the APU produce the required starting air supply.
The APU generates pneumatic power using bleed air extracted between the compressor and combustion chamber.
b] how is the starting air supplied and controlled from the APU to the engine
The starting air is supplied to the engines starters through a network of ducts and valves.
c] describe how the torque from the starter motor is transmitted to the engine rotating assembly.
Usually the starter motor turns the accessory gearbox, which rotates the jet engine shaft.
d] describe the main functions and components of a high energy ignition system.
An high-energy ignition systems obviously rely on igniters which supply electrical sparks in the combustion chamber for starting or maintaining the air/fuel combustion.
The igniters are powered by exciters generating high-voltage pulses from the aircraft electrical power system.
The exciters are controlled by the engine controllers according to the position of the fuel control ; ignition and fire switches.