We have to find area of red portion.
Since side of square = 1, So radius of both the circles = 1.
Since ABCD is an square, so angle at centers of circle = 90.
Area of sector ACD = ($\pi$r$^{2}$)/4.
So area of red portion = 2 * area of sector ACD( equal to sector BCD) - area of square.
= 2 * $\pi$/4 - 1
= $\pi$/2 - 1.
Ans- B.