Answer
The quark makeup of the antineutron is $\bar{u} \bar{d} \bar{d}$
Work Step by Step
The quark makeup of the neutron is $udd$
We can verify that the quark makeup of the antineutron is $\bar{u} \bar{d} \bar{d}$
The antineutron $\bar{n}$ has charge $0$, strangeness $0$, and baryon number $-1$
The antiquark $\bar{u}$ has charge $-\frac{2}{3}$, strangeness $0$, and baryon number $-\frac{1}{3}$
The antiquark $\bar{d}$ has charge $+\frac{1}{3}$, strangeness $0$, and baryon number $-\frac{1}{3}$
The conservation laws show that the quark makeup of the antineutron is $\bar{u} \bar{d} \bar{d}$
