If you keep V/f constant , then field flux remains constant , hence the generated torque now depends only on rotor current , there is a relation of electromagnetic torque generated in motors ,Te = (3/2)*(P/2) *Fs*Fr *sin(delta) , by keeping Fs constant , you remove the coupling between flux and the armature current , hence Torque response will be instantaneous with change in rotor current and response will not get delayed ,with smooth control, so flux vs time should be a parallel line to time axis.
At low frequencies once you try to maintain V/f constant , stator applied voltage also reduces , and hence the stator impedance drop becomes significant , hence flux value reduces from rated . At high frequencies impedance drop is negligible compared to the applied voltage ,so we take V =E .
D. At low frequency, the stator flux remains unchanged at its rated value.