The correct answer is B) Starter. The starter is responsible for accelerating the compressor to the self-sustaining speed of the engine. When the engine is initially started, the starter provides the necessary torque to spin up the compressor to the point where it can self-sustain and continue running on its own. The compressor extension shaft, PT shaft, and mechanical driveshaft are not responsible for this function - they are other components of the engine system that serve different purposes.