Technician A says coils are usually wrapped around an iron core. Technician B says coils block direct current flow. Who is right?

Technician A is correct a coil is normally wound round an iron core.

Technician B is wrong, a coil such as an inductor, allows direct current to flow through it, while blocking alternating current flow

Explanation:

A coil is a conductor of electricity made up of an electric wire wound round a soft iron core. They are used as inductors, electromagnets, transformers and electric motors. Passing current through the coil generates a magnetic field and conversely, turning the coil in a magnetic field generates electric current

In inductors, the coil is used to block AC current while allowing DC current to pass.

Therefore coils allow both AC and DC current to flow through them but they limit AC current flow as the frequency of the AC current increases.