Given: A circle is inscribed in $a$ square. So if the side of Square is $a$, then $a$ is the diameter of circle.
Area of Circle = $\frac{{\pi a^2}}{4}$
Area of Square = $a^2$

Expected outcome is dart to land on the area that's out of circle but still within square, which is = $a^2- \frac{{\pi a^2}}{4}$
and probability of expected is Expected/Total outcome = $(a^2 - \frac{\pi a^2}{4})/a^2= 1 - \frac{\pi}{4}$
Answer is B)