The electric field at a point on the equatorial plane at a distance r from the centre of a dipole having dipole moment $\vec{p}$ is given by,

(r >> separation of two charges forming the dipole, $\epsilon_0$ - permittivity of free space)

(1) $\vec{E}=-{\vec{P}\over4\pi\epsilon_0r^3}$

(2) $\vec{E}={\vec{P}\over4\pi\epsilon_0r^3}$

(3) $\vec{E}={2\vec{P}\over4\pi\epsilon_0r^3}$

(4) $\vec{E}=-{\vec{P}\over4\pi\epsilon_0r^2}$

(r >> separation of two charges forming the dipole, $\epsilon_0$ - permittivity of free space)

(1) $\vec{E}=-{\vec{P}\over4\pi\epsilon_0r^3}$

(2) $\vec{E}={\vec{P}\over4\pi\epsilon_0r^3}$

(3) $\vec{E}={2\vec{P}\over4\pi\epsilon_0r^3}$

(4) $\vec{E}=-{\vec{P}\over4\pi\epsilon_0r^2}$