The correct answer is D) Split the phase to create greater starting torque. The main function of a capacitor in a single-phase motor is to split the phase in order to create greater starting torque. By placing a capacitor in series with the starting winding, it creates a phase shift between the main winding and the starting winding. This phase shift increases the starting torque of the motor, allowing it to overcome the initial load and start running. The other options are incorrect because they do not describe the primary purpose of a capacitor in a single-phase motor. Option A is incorrect as the capacitor does not prolong the life of the starting contacts. Option B is incorrect as the capacitor is used to split the phase, not reduce the phase angle. Option C is incorrect as the capacitor is not used to reduce radio interference in this application.