(d)
Put $x^{2} = \cos{2\theta\ }$in the given equation, we get
$y = \tan^{- 1}\frac{\sqrt{1 + \cos{2\theta}} - \sqrt{1 - \cos{2\theta}}}{\sqrt{1 + \cos{2\theta}} + \sqrt{1 - \cos{2\theta}}}$
$= \tan^{- 1}\frac{\cos{\theta - sin\theta}}{\cos{\theta + \sin\theta}}$
$= \tan^{- 1}{\tan\left( \frac{\pi}{4} - \theta \right)}$
$\Rightarrow \ y = \frac{\pi}{4} - \theta = \frac{\pi}{4} - \frac{1}{2}\cos^{- 1}x^{2}$
$\Rightarrow \ \frac{dy}{dx} = 0 - \frac{1}{2}\left( - \frac{(2x)}{\sqrt{1 - x^{4}}} \right) = \frac{x}{\sqrt{1 - x^{4}}}$