Answer
The quark combination needed to form $\Xi^0$ is $ssu$
Work Step by Step
The $\Xi^0$ has charge $0$, and strangeness $-2$
The strange quark $s$ has charge $-\frac{1}{3}$, and strangeness $-1$
The up quark $u$ has charge $+\frac{2}{3}$, and strangeness $0$
The quark combination needed to form $\Xi^0$ is $ssu$
