In the circle given below with centre o, the angle A is $\frac{\pi}{4}$, what is the length of arc A ? <br>

Question

$\begin{array}{1 1} (a)\; \frac{\pi r}{4} \\ (b)\; \frac{\pi r}{2} \\ (c)\; \frac{\pi r}{8} \\ (d)\; \pi r \end{array}$