The correct answer is: A. Soft magnetic materials
Soft magnetic materials are used for the construction of the core of electromagnets because they have a high permeability, which means that they can be easily magnetized and demagnetized. This is important for electromagnets, which need to be able to be turned on and off quickly and easily.
Hard magnetic materials, on the other hand, have a low permeability and are difficult to magnetize and demagnetize. This makes them unsuitable for use in the core of electromagnets.
Option C is incorrect because it is not always the case that either soft or hard magnetic materials can be used for the construction of the core of electromagnets. The type of material that is used will depend on the specific application.
Option D is incorrect because soft magnetic materials are used for the construction of the core of electromagnets.