Understand the assertion

A polar dielectric has permanent molecular dipole moments. However, in the absence of an external electric field, these dipoles are oriented randomly due to thermal motion.

Therefore, their vector sum over the पदार्थ is: $\vec{P}_{\text{net}} = 0$ on average.

So the statement:

Net dipole moment of a polar linear isotropic dielectric substance is not zero even in the absence of an external electric field

is incorrect.

Understand the reason

The reason says:

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

This is correct.

Conclusion

• Assertion (A): False
• Reason (R): True

Hence the correct option is: $\boxed{\text{C}}$ where

(A) is not correct but (R) is correct.

Compare with stored answer

Stored correct answer = C

Our derived answer = C

So they agree.