Which type of starter requires air from an external low pressure, high volume source?

The correct choice, which identifies the starter that requires air from an external low pressure, high volume source, is the pneumatic starter. This type of starter utilizes compressed air to initiate the engine's operation, making it highly effective for starting gas turbine engines particularly in large aircraft.

Pneumatic starters operate by using the stored energy in compressed air to drive a turbine or a motor that turns the main engine's compressor and initiates the ignition process. The reliance on an external air supply makes them suitable for scenarios where large volumes of air are needed to generate enough torque for engine start-up, especially in environments where electric or hydraulic systems may be less practical or efficient.

This contrasts with other starting systems. For example, electric starters draw power from the aircraft’s battery or electrical system, while hydraulic starters utilize pressurized hydraulic fluid to start the engine. Mechanical starters typically involve manual or gear-driven initiation methods, relying on physical mechanisms rather than fluid or electric systems. Therefore, in the context of needing an external low pressure, high volume air source, the pneumatic starter distinctly matches the criteria.