Explanation

(c) Let an electric dipole AB is situated in a vacuum. Let p be a point at a distance r from the dipole when p is electric dipole moment, then electric field intensity is given by
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$E = \frac{1}{4\pi\varepsilon_{0}}.\frac{2pr}{\left( r^{2} - l^{2} \right)^{2}}$
If $l$ is very small compared to $r(l \ll r),\ $then$\ l^{2}$may be neglected in comparison to $r^{2}$. Then
$E = \frac{1}{4\pi\varepsilon_{0}}.\frac{2p}{r^{3}}NC^{- 1}$
$\Longrightarrow E \propto \frac{1}{r^{3}}.$