We have that $τ(n)=\sum_{d|n} 1$ is the number of dividers of n.
Dividers of $m^n$ are $1,m,m^2,...,m^n$ then we have that $τ(m^n)=n+1$.
Is this correct so far?
Now we must prove that $τ(m^n)$ and $n$ are coprime, and I've though of using Bezouts theorem. Is this also a correct way?
Hint
If a prime $p$ divides $n$ and $n+1$ then it divides $(n+1)-n = 1$ as well.