Product and cos

Geometry Level 3

$\large \displaystyle\prod \limits^{14}_{k=1}\cos \left( \frac{k \pi }{15} \right) =\ ?$

0 14

\frac {1}{2}

\frac {2}{3}

\bigg(\frac {1}{2}\bigg)^{14}

-\bigg(\frac {1}{2}\bigg)^{14}