(c)
Given that, $\angle A = \tan^{- 1}{2,\angle B = \tan^{- 1}3}$
We know that, $\angle A + \angle B + \angle C = \pi$
$\Rightarrow \tan^{- 1}{2 + \tan^{- 1}{3 + \angle C = \pi}}$
$\Rightarrow \tan^{- 1}{\left( \frac{2 + 3}{1 - 2 \times 3} \right) + \angle C = \pi}$
$\Rightarrow \tan^{- 1}{( - 1) + \angle C = \pi}$
$\Rightarrow \frac{3\pi}{4} + \angle C = \pi$
$\Rightarrow \angle C = \frac{\pi}{4}$