(c)
$\lim_{x \rightarrow 0^{+}}\ f(x) = \lim_{x \rightarrow 0^{+}}\ \lambda\lbrack x\rbrack = 0$
$\lim_{x \rightarrow 0^{-}}\ \ f(x) = \lim_{x \rightarrow 0^{-}}\ \ 5^{1/x} = 0$
And $f(0) = \lambda\lbrack 0\rbrack = 0$
$\therefore\ f$ is continuous only whatever $\lambda$ may be