The rotor developed by a single-phase motor at starting is

The correct answer is C. less than the rated torque.

A single-phase motor has a starting torque that is less than its rated torque. This is because the starting current is much higher than the running current, which causes the magnetic field to be weaker. The weaker magnetic field produces less torque, which makes it harder for the motor to start.

Option A is incorrect because the starting torque is not more than the rated torque. Option B is incorrect because the starting torque is not the rated torque. Option D is incorrect because the starting torque is not zero.