Proving regular languages are closed under a form of interleaving
Hint. Because $L_1$ and&nbsp;$L_2$ are both regular, you know they're accepted by NFAs (or DFAs; it doesn't matter) $M_1$ and&nbsp;$M_2$, respectively. To show that $\mathrm{Interleave}\,(L_1,L_2)$ is regular, show that it's accepted by some NFA&nbsp;$M$. For each character it receives, $M$&nbsp;can decide to act either like&nbsp;$M_1$ or like&nbsp;$M_2$.