Explain the operation principle of differential relay

A non–salient pole, Y-connected, three-phase,two-pole synchronous machine has a synchronousreactance of 7Ω  and negligible resistance androtational losses. One point on the open-circuitcharacteristic is given by Vo = 400 V (phase voltage)for a field current of 3.32 A. The machine is to beoperated as a motor, with a terminal voltage of 400 V(phase voltage). The armature current is 50 A, withpower factor 0.85, leading. Determine Eb, fieldcurrent, torque developed, and power angle δ.

Please answer the question below. Thank you so much!!

13. A synchronous machine is run as an unloaded motor. If a low voltage is supplied at the ratedfrequency, and if the full-load armature current flows, a test is being run to determine theA. windage loss.B. I²R loss.C. stray-load loss.D. core loss. Note: I have doubts if it is the letter A or the letter B, can you help me in this question, thanks.