In $\Delta$ABC $\angle A=30^{\large\circ}$
$BC=10$ cm
Let $O$ be the centre of the circle.
$\Rightarrow\:\angle BOC=60^{\large\circ}$
$OB$ and $OC\Rightarrow radius$
$\angle OBC=\angle OCB =60^{\large\circ}$
$\Rightarrow \Delta OBC$ is an equilateral $\Delta$le.
Radius of circle is $OB=OC=BC=10cm$
Area of the circumcircle is $\pi r^2$
$\Rightarrow \pi(10)^2$
$\Rightarrow 100\pi$sq.cm
Hence (a) is the correct answer.