c
The given condition takes place when line of force of mg passes through
$\theta=45^{\circ}$ $...(i)$
and $\operatorname{mg} \sin \theta \leq f_{\mathrm{r} \max }=\mu \mathrm{mg} \cos \theta$
$\Rightarrow \mu \geq \tan \theta$
$\Rightarrow \mu \geq 1$
