Among the following _________ has the highest retentivity.
A) Aluminium
B) Steel
C) Nickel
D) Soft iron.

Hint: Retentivity is defined as the ability of the substance to remain in the magnetic field ever after removal of the external source. The magnetic strength can be represented by the retentivity. When the external magnetic field is decreased to zero after it reaches the saturation point of magnetization then the remaining magnetization is known as the retentivity of that substance.

Complete answer:
It is asked in the problem which of the following substances Aluminium, steel, nickel, soft iron which has highest retentivity and we need to find the correct answer for this problem. The steel retains the magnetic field effect longer than the aluminum, steel, nickel and soft iron and therefore steel has the highest retentivity of the substance among the following given substances.

The correct option for this problem is option B.

Additional information:
The hysteresis is the state which depends upon the present state but also on the past history. The phenomenon in which the flux density is lagging behind the magnetizing force in a magnetic material is known as hysteresis. Coercivity is the magnetizing field which is needed to demagnetize the magnetic material completely. When an external magnetic field is applied on a material like ferromagnetic then the atomic dipole aligns themselves with the external magnetic field and when the magnetic field is removed the alignment stays and the material is therefore magnetized forever.

Note: The students are advised to understand and remember the concept of retentivity as it is very important in solving these problems. The retentivity in the magnetic field is there in the material even after the removal of the external magnetic field.