b
Magnetic moment of the current loop
$A=\pi\left(\frac{a}{2}\right)^{2} \times 2+a^{2}$
$=\left(\frac{\pi}{2}+1\right) \mathrm{a}^{2}$
$\overrightarrow {\text{M}} = {\text{I}}\overrightarrow {\text{A}} = \left( {\frac{\pi }{2} + 1} \right){\text{I}}{{\text{a}}^2}\widehat {\text{k}}$