Assertion : In the absence of an externally applied electric field, the displacement per unit volume of a polar dielectric material is always zero.

Reason : In polar dielectrics, each molecule has a permanent dipole moment but these are randomly oriented in the absence of an externally applied electric field.

Both Assertion and Reason are true but R is not correct explanation of A

D= εoE+ P

vector E = applied electric field and vector P = polarisation or dipole moment per unit volume.

Now in the absence of an electric field (vector E = 0) the polarisation vector P vanishes (because vector p is there if  vector E is there).

Therefore displacement  vector D = 0