b) earths outer iron core
A coil of wire that is carrying a current and produces a magnetic field is called a solenoid. So the correct option is option "B", in regards to the question given. The term solenoid was first invented by the famous French physicist named Andre-Marie Ampere. In the terms of a physicist, it indicates a coil that has a far greater lenth than the diameter and is wrapped around a core made of metal to create a uniform magnetic field. If a compass is brought near the wire through which current is passing, it will indicate the direction in which the current is flowing.