$\cos^4 \theta-\sin^4 \theta=$

Question

$\begin{array}{1 1} (a)\;1-2 \cos ^2 \theta \\ (b)\;2 \cos^2 \theta-1 \\ (c)\;2 \sin ^2 \theta-1 \\ (d)\;\cos^2 \theta-1 \end{array}$