Correct Answer - Option 3 : by OC and ZPF test
Methods of finding Voltage Regulation in Synchronous Generator:
Synchronous impedance method: In the synchronous impedance method voltage regulation is determined by calculating synchronous impedance Zs.
MMF method: It is also used to determining voltage drop through MMF, but the drops in machine IaXL is voltage quantity and IaXa is MMF quantity. Because armature reaction is the reduction of flux which is represented through voltage drop.
Potier Method (ZPF method): In potier method voltage quantities are calculated on a voltage basis and MMF quantities are calculated on an MMF basis.
It separated leakage reactance and armature reactance and named it Potier reactance.
It requires an open circuit, short circuit, and zero power factor tests. But to find only leakage reactance open circuits and zero power factor tests are sufficient.