(d)
Given, $\sin{\left( \frac{\pi}{4}\cot\theta \right) = \cos\left( \frac{\pi}{4}\tan\theta \right)}$
$\Rightarrow \ \sin{\left( \frac{\pi}{4}\cot\theta \right) = \sin\left( \frac{\pi}{2} - \frac{\pi}{4}\tan\theta \right)}$
$\Rightarrow \frac{\pi}{4}\left( \tan\theta + \cot\theta \right) = \frac{\pi}{2}$
$\Rightarrow \left( \tan\theta - 1 \right)^{2} = 0\ \ \ \Rightarrow \ \ \tan\theta = 1 = \tan\frac{\pi}{4}$
$\therefore\ \ \ \theta = n\pi + \frac{\pi}{4}$