The correct answer is: A. Reduce eddy current loss.
Eddy currents are induced currents that flow in a conductor when it is placed in a changing magnetic field. These currents can cause significant losses in AC circuits, as they dissipate energy as heat. Laminated iron is used in AC circuits to reduce eddy current losses. The thin layers of iron act as individual inductors, and the eddy currents are confined to each layer. This reduces the overall eddy current losses in the circuit.
Option B is incorrect because increasing heat radiation would not reduce eddy current losses. In fact, it would likely increase them, as the heat would cause the iron to expand and become less resistive.
Option C is incorrect because making assembly cheap and easier is not the primary purpose of using laminated iron in AC circuits. While this may be a secondary benefit, it is not the main reason for using laminated iron.
Option D is incorrect because reducing circuit permeability would not reduce eddy current losses. In fact, it would likely increase them, as the magnetic field would be less concentrated in the iron.
In conclusion, the correct answer is: A. Reduce eddy current loss.