(a)
Since, $\lim_{x \rightarrow 0}\ \ f(x) = f(0)$
$\Rightarrow \ \lim_{x \rightarrow 0}\frac{\sin{\pi x}}{5x} = k\$
$\Rightarrow \ \ (1)\frac{\pi}{5} = k\ \ \ \Rightarrow \ \ \ k = \frac{\pi}{5}\ \ \$ $\left\lbrack \because\ \ \ \lim_{x \rightarrow 0}\ \frac{\sin x}{x} = 1 \right\rbrack$