The single phasing relays are used for the protection of

single phase motors only
two phase motors only
two single phase motors running in parallel
three phase motors

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.