For an electromagnet the core should have
(A) High retentivity and low coercivity.
(B) Low retentivity and high coercivity.
(C) High retentivity and high coercivity.
(D) Low retentivity and low coercivity.

Hint: To solve this type of problem we should know about the property of electromagnet which will decide the nature of material to be used for it as well as we should know about the hysteresis curve of different materials and material’s different electromagnetic properties.
Electromagnet: It is a type of magnet which is made of coil rounded around a metallic strip or rod. Magnetic field is generated when electric current will pass through them and it behaves like a magnet.

 Complete Step by Step solution:
To choose material for the core of an electromagnet first you should know about the property of electromagnet.
An electromagnet must possess following property:
1. Low retentivity for easy demagnetisation.
2. It should have low coercivity
3. It should not get heated up during magnetisation and demagnetisation so hysteresis loss must be small
4. High permeability
Retentivity: The property of the magnetic material to retain magnetism even when applied magnetizing force is removed. It must be high so that an electromagnet can produce a more effective magnetic field for more time.
Coercivity: It is the resistance produced by the magnetic material to change the magnetization. This must be low because if it is high then it is difficult to control the magnetic field generated by the electromagnet by adjusting the electric field.
So, the answer is option C that is, an electromagnet core must have high retentivity and low coercivity.

Note:
Hysteresis curve: It shows the relationship between the induced magnetic flux density and the magnetizing force which shows material magnetic property when it is continuously applied to varying magnetizing force.
Electromagnets are used in transformers, motors, generators which are used in our daily life.