If $\cos(\theta-\alpha)=a$ and $\sin(\theta-\beta)=b$ then $\cos^2(\alpha-\beta)+2ab\sin(\alpha-\beta)$ is equal to

Question

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