Solution:Concept: Magnetic keeper is used to maintain and preserve the strength of a magnet. Every thing has a property named as coercivity .which helps them to save the magnetic strength.
The poles of the magnets become weaker by time. Therefore, magnets need to be reserved and this can be done by using the Magnetic keepers.
Magnetic keepers are used to reserve the strength of a magnet. They are made up of magnetically iron . Magnets having low coercivity are reserved by these magnetic keepers. External fields can easily affect the domains of the magnets that have low coercivities.
Hence, the correct option 4.