The correct answer is: A. single phase motors only.
A single phasing relay is a type of electrical relay that is used to protect single phase motors from the effects of single phasing. Single phasing occurs when one of the three phases of a three phase motor is lost. This can cause the motor to overheat and fail. A single phasing relay will detect the loss of phase and shut off the motor to prevent damage.
Option B is incorrect because two phase motors do not have a single phasing problem. Option C is incorrect because two single phase motors running in parallel are not subject to single phasing. Option D is incorrect because three phase motors are not subject to single phasing.