If $\theta=\large\frac{\pi}{2^n+1}$ then $\cos \theta\cos 2\theta\cos 2^2\theta......\cos2^{n-1}\theta$ is equal to

Question

$(a)\;\large\frac{1}{2^n}\normalsize\qquad(b)\;\cos\theta\qquad(c)\;2\qquad(d)\;2^n$