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Assertion : In the absence of an external electric field, the dipole moment per unit volume of a polar dielectric is zero.
Reason : The dipoles of a polar dielectric are randomly oriented.
If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
If Assertion is correct but Reason is incorrect.
If both the Assertion and Reason are incorrect.
Solution
$\vec{D}=\varepsilon_{0} \vec{E}+\vec{P}$ where
vector $\mathrm{E}=$ applied electric field and
vector $\mathrm{P}=$ polarisation or dipole moment per unit volume
$=n \vec{p} \times 2$
[vector $\mathrm{P}$ is the component of dipole moment of each molecule in direction vector $E$ and $n$ is the no. of molecules per unit volume]
Now in the absence of an electric field (vector $E=0$ ) the polarisation vector $P$ vanishes (because vector $\mathrm{p}$ is there if vector $\mathrm{E}$ is there).
Therefore displacement vector $\mathrm{D}=0$
