(d)
$\frac{\cos x}{1 + \sin x} = \frac{\cos^{2}{\frac{x}{2} - \sin^{2}\frac{x}{2}}}{\left( \cos{\frac{x}{2} + \sin\frac{x}{2}} \right)^{2}}$
$= \frac{\cos{\frac{x}{2} - \sin\frac{x}{2}}}{\cos{\frac{x}{2} + \sin\frac{x}{2}}}\ \ \ \ = \frac{1 - \tan\frac{x}{2}}{1 + \tan\frac{x}{2}}$
=$\ \tan\left( \frac{\pi}{4} - \frac{x}{2} \right)$
$\therefore = \tan^{- 1}{\left( \tan\left( \frac{\pi}{4} - \frac{x}{2} \right) \right) = \frac{\pi}{4} - \frac{x}{2}\ }$
$\Rightarrow \ \ \ \ \ \ \ \frac{dy}{dx} = - \frac{1}{2}$