If $u=\sqrt{a^2\cos^2\theta+b^2\sin^2\theta}+\sqrt{a^2\sin^2\theta+b^2\cos^2\theta}$ then the difference between the maximum and minimum values of $u^2$ is given by

Question

$\begin{array}{1 1}(a)\;(a-b)^2&(b)\;2\sqrt{a^2+b^2}\\(c)(a+b)^2&(d)\;2(a^2+b^2)\end{array}$