What is the magnetic field at the center of the circular current loop?

Question

$\begin{array}{1 1} (a)\;B=\large\frac{\mu_0I}{2 \pi}\\ (b)\;B=\large\frac{\mu_0I}{2 \pi r} \\ (c)\;B=\large\frac{\mu_0I}{2 r} \\ (d)\;I=\frac{\mu_0I}{\pi} \end{array}$