Answer
The strangeness of the anitneutron is $0$
Work Step by Step
The $\pi^+$ has strangeness $0$
The proton $p$ has strangeness $0$
Let $S$ be the strangeness of the neutron $\bar{n}$
Then according to the strangeness conservation law:
$(0)+(0) = (0)+(0)+S$
$S = 0$
The strangeness of the anitneutron is $0$.
