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