A magnetic needle suspended horizontally by an unspun silk fibre oscillates in the horizontal plane, because of the restoring force originating mainly from
A. The torsion of the silk fibre
B. The horizontal component of earth’s magnetic field
C. The force of gravity
D. All of the above

Hint: Firstly, we should know about magnetic fields. The area in which the force of magnetism acts around a magnetic material or a moving electric charge is known as the magnetic field. As parts of the electromagnetic force, one of the four fundamental forces of nature, magnetic field and electric field are interdependent.

Complete step by step solution:
Working of magnetic compass: The earth is thought of as a giant bar magnet with its North and South poles aligned with the geographic South and North Poles, respectively. As a result, the magnetic needle's North pole in a magnetic compass is drawn to the geographic North Pole of the earth, while the South pole is drawn to the geographic South Pole. As a result, the magnetic compass's needle always points in the direction of North and South.

Even in the absence of any other magnets, a freely suspended magnet always faces north and south. This argues that the Earth itself functions as a magnet, causing a freely suspended magnet to always point in one of two directions: north or south. The magnetic field of the Earth is shaped like a bar magnet with a length of one-fifth of the diameter of the Earth buried in its centre.

Hence, option B is correct.

Note: A magnetic compass is essentially a needle constructed of a magnetic material that responds to magnetic fields by moving. The needle is positioned such that it can freely move along the vertical axis in the horizontal plane. The needle is kept in a container with a glass lid that is transparent.