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 $\mathrm{D}=0$