(c)
Let $I = \frac{d}{dx}\begin{Bmatrix}
\tan^{- 1}{\left( \frac{2x}{1 - x^{2}} \right) + \tan^{- 1}\left( \frac{3x - x^{3}}{1 - 3x^{2}} \right)} \\
- \tan^{- 1}\left( \frac{4x - 4x^{3}}{1 - 6x^{2} + x^{4}} \right)
\end{Bmatrix}$
Put $x = \tan{\theta\ }$the given equation
$\therefore\ \ \ \ I = \frac{d}{dx}\{\tan^{- 1}{(\tan{2\theta) + \tan^{- 1}{(\tan{3\theta) - \tan^{- 1}{(\tan{4\theta)\}}}}}}}$
$= \frac{d}{dx}(\theta) = \frac{d}{dx}(\tan^{- 1}{x) = \frac{1}{1 + x^{2}}}$