Answer
The corresponding particle is an antineutron $\bar{n}$
Work Step by Step
The proton $p$ has strangeness $0$, charge $+1$, and baryon number $+1$
The antiproton $\bar{p}$ has strangeness $0$, charge $-1$, and baryon number $-1$
The left side of the process has net strangeness $0$, net charge $0$, and net baryon number $0$
The neutron $n$ has strangeness $0$, charge $0$, and baryon number $+1$
According to the conservation laws, the $x$ must have strangeness $0$, charge $0$, and baryon number $-1$
The corresponding particle is an antineutron $\bar{n}$
