a b c d

c

$\text{Electric Field due to dipole along x axis at (a,0) is } E_1 = \frac{2p}{4\pi \epsilon_0 a^3}$ $\text{Electric Field due to dipole along y axis at (a,0) is } E_2 = \frac{p}{4\pi \epsilon_0 a^3}$ $\text{ Both the electric field are mutually perpendicular, resultant Electric field is } E = \sqrt {E_1^2 + E_2^2} = \frac{\sqrt 5p}{4\pi \epsilon_0 a^3}$