The correct answer is: D. saturation of core.
A transformer is a device that transfers electrical energy from one circuit to another without a change in frequency. It does this by using electromagnetic induction to create a magnetic field that induces a voltage in a second circuit.
The main reason for generation of harmonics in a transformer is saturation of the core. When a transformer is carrying a large load, the magnetic field becomes very strong. This can cause the core to become saturated, which means that it can no longer hold as much magnetic flux. This can lead to a number of problems, including increased core losses, decreased efficiency, and increased harmonic distortion.
Fluctuating load can also cause harmonics, but this is usually not a major issue. Poor insulation can also cause problems, but these are usually related to insulation breakdown and not to harmonic distortion. Mechanical vibrations can also cause problems, but these are usually related to noise and vibration, not to harmonic distortion.
In conclusion, the main reason for generation of harmonics in a transformer is saturation of the core.