Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A) : Net dipole moment of a polar linear isotropic dielectric substance is not zero even in the absence of an external electric field.

Reason (R) : In absence of an external electric field, the different permanent dipoles of a polar dielectric substance are oriented in random directions.

In the light of the above statements, choose the most appropriate answer from the options given below :

A polar dielectric molecule (for example $$H_2O$$) possesses a permanent dipole moment $$\vec p_{0}$$ even when no external field is present.

However, in a macroscopic sample of such a dielectric, every molecule can point in any direction with equal probability when there is no external electric field. Hence the molecular dipoles are randomly oriented.

When vectors of equal magnitude are randomly oriented, their vector (vectorial) sum averages to zero: $$\vec P_{\text{net}} = \sum_i \vec p_{0i} = \vec 0$$.

Therefore the bulk or net dipole moment of the whole specimen is zero in the absence of an external electric field.

Now examine the statements:
Assertion (A): Net dipole moment is not zero without the field — this contradicts the conclusion above, so (A) is incorrect.
Reason (R): Permanent dipoles are oriented randomly when there is no field — this is exactly what happens, so (R) is correct.

Because (A) is false while (R) is true, (R) cannot be the correct explanation of (A). Thus the correct choice is Option D: $$(A)\ \text{is not correct but}\ (R)\ \text{is correct}$$.