Find the difference between the largest $5$-digit number and the smallest $6$-digit number.

We have,

Largest $5$-digit number$=99999$

Smallest $6$-digit number.$=100000$

Difference between the largest $5$-digit number and the smallest $6$-digit number :

$$\begin{gathered} =100000-999999 \\ =1 \end{gathered}$$

Hence, $1$ is the correct answer.