If $\cos^{-1}x-\cos^{-1}\large\frac{y}{2}$$=\alpha$ then $4x^2-4xy.\cos \alpha-y^2$ is equal to

Question

$(a)\;-4\sin^2\alpha\qquad(b)\;4\sin^2\alpha\qquad(c)\;4\qquad(d)\;2\sin^2\alpha$