The correct answer is B. 1.5 to 2.0 A/mm2.
Current density is the current per unit area of a conductor. It is a measure of how much current is flowing through a conductor per unit of its cross-sectional area. The higher the current density, the more heat is generated in the conductor. This can lead to overheating and damage to the conductor.
The current density in a transformer is determined by the type of transformer, the size of the transformer, and the operating conditions of the transformer. For large power transformers, self oil cooled type or air blast type, the current density is typically in the range of 1.5 to 2.0 A/mm2. This is because these transformers are designed to handle high currents and they have a large surface area to dissipate heat.
Option A is incorrect because it is too low. Option C is incorrect because it is too high. Option D is incorrect because it is not a valid range of current densities.