Introduction:
The no-load primary input of a transformer refers to the power consumed by the transformer when there is no load connected to its secondary winding. This power is mainly consumed by the core losses or iron losses of the transformer. Iron losses are primarily composed of hysteresis loss and eddy current loss.

Explanation:
When a transformer is operated with no load, the secondary winding is open-circuited, and the current flowing through the primary winding is very small. This small primary current is mainly used to overcome the core losses and magnetize the core.

The core losses in a transformer are caused by two main factors:

1. Hysteresis Loss: Hysteresis loss occurs due to the cyclic magnetization and demagnetization of the transformer core. This loss is caused by the reversal of magnetic domains within the core material, resulting in energy dissipation in the form of heat.

2. Eddy Current Loss: Eddy current loss is caused by the circulating currents induced in the core material due to the varying magnetic field. These circulating currents create localized magnetic fields, which in turn generate heat in the core material.

Iron Loss Calculation:
The iron loss or core loss can be calculated by adding the hysteresis loss and eddy current loss. The iron loss is usually measured using a wattmeter connected to the primary side of the transformer.

Conclusion:
In conclusion, the no-load primary input of a transformer is mostly consumed by the iron losses or core losses. These losses are primarily composed of hysteresis loss and eddy current loss. The copper losses, which are primarily caused by the resistive heating of the transformer windings, are negligible during no-load conditions. Therefore, option 'A' - Iron loss of transformer is the correct answer.