(c)
Since, $f(x)$ is continuous at $x = \frac{\pi}{2}$
$\lim_{x \rightarrow \frac{\pi^{- 1}}{2}}\ \ (mx + 1) = \lim_{x \rightarrow \frac{\pi^{+}}{2}}\ \ \ \ \left( \sin{x + n} \right)$
$\Rightarrow \ \ \ \ m\frac{\pi}{2} + 1 = \sin{\frac{\pi}{2} + n}$
$\Rightarrow \ \ \ \ \frac{m\pi}{2} = n$