The correct answer is C) Split the phase to create greater starting torque. In a single-phase motor, a capacitor is used in the starting circuit to create a phase shift between the main winding and the starting winding. This phase shift helps to split the single-phase input into two out-of-phase currents, which generates a greater starting torque for the motor. The increased starting torque allows the motor to overcome the initial inertia and begin rotating. The other options are incorrect because: A) Reducing radio interference is not the main function of a capacitor in a single-phase motor; B) Prolonging the life of the starting contacts is not the primary purpose of the capacitor; and D) Reducing the phase angle is not the key role of the capacitor, as the goal is to create a phase shift, not reduce it.