d
At $V_{\max }, f$ will be limiting in nature.
$\therefore$ Balancing force in vertical direction,
$N \cos 30^{\circ}-{mg}-\mu {N} \cos 60^{\circ}=0$
$\Rightarrow {N}\left[\cos 30^{\circ}-\mu \cos 60^{\circ}\right]={mg}$
$\therefore {N}=\frac{800 \times 10}{(0.87-0.1)} \approx 10.2 \times 10^{3} {kgm} / {s}^{2}$
