15) Magnetic field induction at the centre O of a square loop of side 'a' carrying current I as shown in figure is.
$$\begin{gathered} \left(a\right) \frac{{\mu }_{0}I}{\sqrt{2}\mathrm{\pi a}} \\ \left(\mathrm{b}\right) 2\sqrt{2}\frac{{\mathrm{\mu }}_0\mathrm{I}}{\mathrm{\pi a}} \\ \left(\mathrm{c}\right) \frac{2{\mathrm{\mu }}_0\mathrm{I}}{\mathrm{\pi a}} \\ \left(\mathrm{d}\right) \mathrm{zero} \end{gathered}$$