Answer
The charge quantum number of the anitneutron is $0$
Work Step by Step
The $\pi^+$ has charge $+1$
The proton $p$ has charge $+1$
Let $q$ be the charge of the antineutron $\bar{n}$
Then according to the charge conservation law:
$(+1)+(+1) = (+1)+(+1)+q$
$q = 0$
The charge quantum number of the anitneutron is $0$.
