The correct answer is D. silicon steel.
Silicon steel is a type of electrical steel that is used in the construction of transformers. It is a ferromagnetic material that has a high permeability and low coercivity. This makes it ideal for use in transformers, as it allows for a high flux density and low losses.
Wood is a non-magnetic material and would not be suitable for use in a transformer. Copper and aluminium are both good conductors of electricity, but they are not ferromagnetic and would not be suitable for use in a transformer core.
Silicon steel is a relatively inexpensive material that is easy to work with. It is also a durable material that can withstand the high temperatures and magnetic fields that are present in transformers.