(c)
Let $y = \frac{\tan^{2}{2x - \tan^{2}x}}{1 - \tan^{2}{2x\ tan^{2}x}}$
$= \frac{(\tan{2x - \tan{x)}}}{(1 + \tan{2x\tan{x)}}}.\frac{(\tan{2x + \tan x})}{(1 - \tan{2x\tan x)}}$
$= \tan{(2x - x)\tan{(2x + x)}}$
$= \tan{x\tan\ 3x}$
On differentiating w.r.t. $x,\$we get
$\frac{d}{dx}(y.\cot{3x) = \frac{d}{dx}(\tan{x\tan{3x\cot{3x)}}}}$
$= \frac{d}{dx}(\tan{x)}$
$= \sec^{2}x$