Calculate the potential energy of the system shown in figure

Question

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