If we consider that the figure is a square and the white region of the figure is a circle, the area of the shaded region is:
\(S=S_{square}-S_{circle}=\ell^2-\pi r^2=10^2-\pi5^2=100-25\pi\\\\
S\approx100-25\cdot3.14=100-78.5\\\\
\boxed{S\approx21.5~cm^2}\Longrightarrow Alternative~D\)
