Find the magnitude of net electric field at the centre of square having charges arranged as shown in figure

Question

$\begin{array}{1 1}(a)\;4 \sqrt{2}\;\large\frac{k\;q}{a^2}\qquad(b)\;2 \sqrt{2}\;\large\frac{k\;q}{a^2}\qquad(c)\; \sqrt{2}\;\large\frac{k\;q}{a^2}\qquad(d)\;\large\frac{k\;q}{a^2}\end{array}$