S and L are the smallest and the largest $n$-digit natural numbers respectively. $L-S$ is always divisible by

For an $n$-digit number, $S=10^{n-1}$ and $L=10^n-1$. So $L-S=(10^n-1)-10^{n-1}=9\cdot 10^{n-1}-1$, which is one less than a multiple of 9 and therefore not divisible by 9. It also ends in 8 or 9 depending on $n$, so it is not divisible by 10. Therefore the correct option is none of these.