What is a dielectric?

The function of a dielectric in a capacitor is

Explain the difference in the behaviour of a conductor and a dielectric in the presence of an external electric field.

Due to which phenomenon does the surface charge density arise on the surface of a dielectric slab when it is placed in a uniform electric field?

What are polar molecules and non-polar molecules? Give examples of both.

$Assertion \; (A)$: Non-polar materials do not have any permanent dipole moment.
$Reason \; (R)$: When a non-polar material is placed in an electric field,the centre of the positive charge distribution of its individual atom or molecule coincides with the centre of the negative charge distribution.
In the light of the above statements,choose the most appropriate answer from the options given below: