Answer
$\pi/4$
Work Step by Step
We know that $\text{probability}=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}$, which here is: $\frac{Area \ A}{Area \ of \ unit \ square}=\frac{r^2\pi/4}{s^2}=\frac{1^2\pi/4}{1^2}=\pi/4$
