The electrostatic potential due to the charge configuration at point P as shown in figure is $\;(b < < a)$

Question

$(a)\;\large\frac{2\;k\;q}{a}\qquad(b)\;\large\frac{2\;k\;q\;b^2}{a^3}\qquad(c)\;\large\frac{k\;q\;b^2}{a^3}\qquad(d)\;zero$