The correct answer is: C. core flux remains practically constant.
The iron loss in a transformer is due to the eddy currents and hysteresis losses in the core. The eddy currents are caused by the changing magnetic field in the core, and the hysteresis losses are caused by the reversal of the magnetic field in the core. The eddy currents and hysteresis losses are proportional to the square of the flux density in the core.
The flux density in the core is proportional to the applied voltage and the turns ratio of the transformer. The applied voltage is constant, and the turns ratio is also constant in an actual transformer. Therefore, the flux density in the core is practically constant from noload to fullload.
The other options are incorrect because:
- The value of transformation ratio remains constant because the turns ratio is fixed in an actual transformer.
- The permeability of transformer core remains constant because the core material is chosen to have a high permeability.
- The primary voltage remains constant because the applied voltage is constant.