What is the length of each side of a square, if its diagonal is 15 ?

Question

$\begin{array}{1 1} (a)\; \frac{16}{\sqrt{2}} \\ (b)\; \frac{15}{\sqrt{2}} \\ (c)\; \frac{13}{\sqrt{2}} \\ (d)\; \frac{14}{\sqrt{2}}\end{array}$