Answer
(b) 2.5
Work Step by Step
The formula to be used here is $s=r\theta$ where $s$ is the arc length, $r$ is the radius of the circle and $\theta$ is the angle subtended at the center of the circle.
Since we need to find $\theta$, $s=r\theta$ is rearranged to $\theta=\frac{s}{r}$. Substituting the values of $s$ and $r$ in the formula:
$\theta=\frac{s}{r}=\frac{20}{8}=2.5$ radian
Therefore, the answer is option (b).