(c)
If function $f(x)$ is continuous at $x = 0$, then
$f(0) = \lim_{x \rightarrow 0}{f(x)}$
$\therefore\ \ \ f(0) = k = \lim_{x \rightarrow 0}{x\sin\frac{1}{x}}$
$\Rightarrow k = 0$ $\left\lbrack \because - 1 \leq \sin\frac{1}{x} \leq 1 \right\rbrack$