Apply Pythagoras Theorem,
\(4r^2 = l^2+b^2\)
Area \(= lb\)
Perimeter, \(P = 2(l+b)\)
\(\frac p2 = l+b\)
Squaring both side,
\(\frac {p^2}{4} = l^2+b^2+2lb\)
\(\frac {p^2}{4} = 4r^2+2lb\)
\(\frac {p^2}{8} - 2r^2 = lb\)
Then, area of the rectangle = \(\frac {p^2}{8} - 2r^2\)
So, the correct option is (B): \(\frac {p^2}{8} - 2r^2\)