A synchronous motor on load draws a current at a leading power factor angle ϕ. If the internal power factor angle which is the phase angle between the excitation emf and the current in the time phasor diagram is ψ. Then the air gap excitation mmf lags the armature mmf by

Correct option is C

Causes of voltage drop:
 Drop due to armature resistance, la Ra drop
 Drop due to leakage reactance, Ia XL drop
 Drop due to armature reaction, Ia Xa drop
Synchronous reactance XS=XL+ Xa
Synchronous impedance Zs=Ra+jXs

Where,
E is the induced EMF
Ia is the armature current
ψ is the angle between E and Ia also called an internal power factor angle.
ψ=ϕ±δ
Where,
' +' is for lagging load
' - ' is for leading load
Air gap voltage
For synchronous motor:
The air gap excitation mmf lags the armature mmf by π/2-ψ
For synchronous Alternator:
The air gap excitation mmf lags the armature mmf by π/2+ψ