The hum in a transformer is mainly attributed to

The correct answer is: C. magnetostriction.

Magnetostriction is the property of certain materials to change their shape when placed in a magnetic field. This change in shape can cause the material to vibrate, which can produce a humming sound.

Load changes can also cause a transformer to hum, but this is usually not the main cause. The oil in a transformer does not usually cause it to hum, unless there is a problem with the transformer. Mechanical vibrations can also cause a transformer to hum, but this is usually not the main cause.

In conclusion, the main cause of the hum in a transformer is magnetostriction.