In a doubly excited magnetic system of salient pole type stator and rotor, the reluctance torque is present only when _____________

A. both stator and rotor currents are acting

B. stator current is acting alone

C. rotor current is acting alone

D. any of the stator or rotor currents acting alone

I have been asked this question in an online interview.

My question is from Doubly Excited Magnetic Systems in chapter Basic Concepts in Rotating Machines of Electrical Machines

The correct answer is D. any of the stator or rotor currents acting alone

For explanation I would say: Equation for magnetic torque is Te=1/2is^2dLs/dθr+1/2ir^2dLr/dθr+isirdMsr/dθr, if ir=0, Te=1/2is^2dLs/dθr and if is=0, then Te = 1/2ir^2dLr/dθr, and these equations for torque are called reluctance torques.