jmc

Assume that the area of each square is $1$. Then, the area of the bolded region in the top left square is $\frac{1}{4}$. The area of the top right bolded region is $\frac{1}{8}$. The area of the bottom left bolded region is $\frac{3}{8}$. And the area of the bottom right bolded region is $\frac{1}{4}$. Add the four fractions: $\frac{1}{4}+\frac{1}{8}+\frac{3}{8}+\frac{1}{4}=1$. The four squares together have an area of $4$, so the percentage bolded is $\frac{1}{4}\cdot 100=\boxed{25}$.