The correct answer is: A. hot because of heavy iron losses taking place in it due to high flux density
When the secondary of a current transformer is open-circuited, there is no current flowing through the secondary winding. This means that there is no opposing magnetic field to the magnetic field produced by the primary winding. The result is that the magnetic field produced by the primary winding becomes very strong, and this can cause the iron core to become hot due to eddy current and hysteresis losses.
Eddy current losses are caused by the movement of electric currents in the iron core. These currents are induced by the changing magnetic field produced by the primary winding. The eddy currents flow in closed loops, and they cause the iron core to heat up.
Hysteresis losses are caused by the magnetic domains in the iron core constantly changing direction as the magnetic field changes. This process requires energy, and this energy is lost as heat.
The combination of eddy current and hysteresis losses can cause the iron core of a current transformer to become very hot if the secondary is open-circuited. This is why it is important to always connect a load to the secondary winding of a current transformer.