Definition of Polarisation:

Polarisation refers to the phenomenon in which the electric dipole moments of individual molecules or atoms align in a particular direction under the influence of an external electric field. This alignment creates a separation of positive and negative charges within the material, resulting in the generation of an electric dipole moment per unit volume.

Explanation:

1. Orientation of Dipoles in Random Direction:
This choice refers to the random orientation of dipoles within a material. In the absence of an external electric field, the dipoles will be randomly oriented, resulting in a net dipole moment of zero. Therefore, this option does not accurately define polarisation.

2. Electric Dipole Moment per Unit Volume:
This option correctly defines polarisation. When an external electric field is applied to a material, the dipoles within the material align themselves in the direction of the field. This alignment creates a separation of positive and negative charges, resulting in the generation of an electric dipole moment per unit volume. This dipole moment per unit volume is referred to as polarisation.

3. Orientation of Dipole Moments:
This choice is similar to the first option and does not accurately define polarisation. The orientation of dipole moments is a consequence of polarisation, rather than the definition of polarisation itself.

4. Change in Polarity of Every Dipole:
This option does not accurately define polarisation. The polarisation phenomenon does not necessarily involve a change in polarity of every dipole. Instead, it involves the alignment of existing dipole moments in a particular direction.

Conclusion:

The best definition of polarisation is the orientation of electric dipole moments per unit volume. This alignment of dipoles under the influence of an external electric field creates a separation of charges within the material and generates an electric dipole moment per unit volume.