The electric field on the axis of a dipole of dipole moment $\;\overrightarrow{P}\;$ at a distance x from the line joining the charges and $\;(x > > d)\;$

Question

$(a)\;\large\frac{k\overrightarrow{P}}{x^3}\qquad(b)\;-\large\frac{k\overrightarrow{P}}{x^3}\qquad(c)\;\large\frac{2 k\overrightarrow{P}}{x^3}\qquad(d)\;-\large\frac{2 k\overrightarrow{P}}{x^3}$