The correct answer is C. both the speed and rotor flux.
The stator back emf is induced by the magnetic field produced by the rotor current. The rotor current is proportional to the rotor flux, which is in turn proportional to the speed of the motor. Therefore, the stator back emf is proportional to both the speed and rotor flux.
Option A is incorrect because the load on the motor does not affect the stator back emf. The load only affects the current that flows through the motor, which does not affect the stator back emf.
Option B is incorrect because the d.c. excitation only affects the rotor flux. The rotor flux is proportional to the d.c. excitation, but the stator back emf is not affected by the d.c. excitation.
Option D is incorrect because the stator back emf does depend on both the speed and rotor flux.