Why is the transformer of an SMPS smaller in size when compared to a transformer in a linear power supply of similar rating?

Correct option is B

In SMPS, the transformer operates at much higher frequencies (typically tens or hundreds of kHz) compared to the 50 or 60 Hz used in linear power supplies. High-frequency operation allows the transformer to be smaller and lighter because the core material can handle the high-frequency magnetic flux more efficiently, reducing the size of the transformer required for the same power output.