The effective resistance of an iron-cored choke working on ordinary supply frequency is more than its true resistance because of

The correct answer is A. Iron loss in core.

The effective resistance of an iron-cored choke working on ordinary supply frequency is more than its true resistance because of iron loss in core. This is because the alternating current flowing through the coil causes the core to alternately magnetize and demagnetize. This process causes eddy currents to flow in the core, which dissipate energy as heat. This heat causes the core to expand and contract, which further increases the eddy currents. This process can lead to a significant increase in the effective resistance of the choke.

The other options are incorrect because:

• Skin effect is a phenomenon that occurs in conductors carrying alternating current. It causes the current to flow mostly on the surface of the conductor, rather than throughout the entire cross-section. This can lead to an increase in the resistance of the conductor, but it is not a significant factor in iron-cored chokes.
• Increase in temperature can cause an increase in the resistance of any conductor, but it is not a significant factor in iron-cored chokes. This is because the core is typically made of a material with a high resistivity, such as iron. The increase in resistance due to temperature is therefore relatively small.
• Capacitive effect between adjacent coil turns is a phenomenon that occurs in coils with a high number of turns. It causes the coil to act like a capacitor, which can lead to a decrease in the effective resistance of the coil. However, this effect is not significant in iron-cored chokes, which typically have a low number of turns.