If a transformer is switched on to a voltage more than the rated voltage

The correct answer is: A. its power factor will deteriorate.

When a transformer is switched on to a voltage more than the rated voltage, the current drawn by the transformer will increase. This is because the impedance of the transformer is inversely proportional to the voltage. The increased current will cause the transformer to heat up, and the increased heat will cause the power factor to deteriorate.

Option B is incorrect because the power factor will decrease, not increase. Option C is incorrect because the power factor will be affected, not remain unaffected. Option D is incorrect because the power factor will not be zero.