D. to provide all the above features.

A. to provide coupling between primary and secondary: The iron core provides a path for the magnetic field to flow through, which connects the primary and secondary windings and allows energy to be transferred from one winding to the other.

B. to increase the magnitude of mutual flux: The iron core increases the strength of the magnetic field, which in turn increases the magnitude of the mutual flux linking the primary and secondary windings.

C. to decrease the magnitude of magnetizing current: The iron core helps to focus the magnetic field, which reduces the amount of current required to establish the magnetic field, thus decreasing the magnetizing current.

D. to provide all the above features: All the above-mentioned points (A, B, C) are the features and the iron core provides all these features.