Magnetic keepers are used to protect magnets from

Answer: Option C

In the domain of Electricity and Magnetism, magnetic keepers are utilized to protect magnets from self-demagnetisation.

When a permanent magnet is exposed to external influences or stored over time without magnetic keepers, it may undergo self-demagnetisation, resulting in a reduction of its magnetic strength. This effect occurs due to various factors such as the alignment of magnetic domains within the magnet gradually becoming disordered.

Magnetic keepers, also known as magnetic shunts or armatures, are ferromagnetic materials (usually soft iron) attached to the poles of a magnet. They serve to complete the magnetic circuit and help maintain the alignment of magnetic domains within the magnet, thus preventing self-demagnetisation.

While magnets can also be affected by external factors like the Earth's magnetic field (Option A), the effect of other magnets (Option B), and demagnetisation due to heating (Option D), magnetic keepers specifically address the issue of self-demagnetisation.

Therefore, Option C: self-demagnetisation is the reason why magnetic keepers are used to protect magnets in the realm of Electricity and Magnetism.