Suppose $m$ belongs to $\Bbb{Z}_n$ and $q$ belongs to $\Bbb{Z}_n$, show $q$ in $c=(m + q) \bmod n$ is unique.
i.e.
Suppose you are given $m$ and $c$, show that $q$ is unique.
Suppose $m$ belongs to $\Bbb{Z}_n$ and $q$ belongs to $\Bbb{Z}_n$, show $q$ in $c=(m + q) \bmod n$ is unique.
i.e.
Suppose you are given $m$ and $c$, show that $q$ is unique.
It's because $\mathbb{Z}/n$ is a group under addition. In particular every element has an additive inverse: if $[a]$ denotes the congruence class (modulo $n$) of the integer $a$, then $[a]+[-a]=[0]$. If $[c]=[m]+[q]$ (where $c,m$ and $q$ are integers) adding $[-m]$ to both sides gives
$[c]+[-m]=[q]+[m]+[-m]=[q]+[0]=[q]$
So the congruence class $[q]$ is uniquely determined in $\mathbb{Z}/n$, given by $[q]=[c-m]$
If you meant that the representative integer $q$ is unique, then that is not true. If we assume that $c\equiv m+q\pmod n$, then $c\equiv m+(q+kn) \pmod n$ for any $k\in\mathbb{Z}$