Answer
$D^0=c\overline{u}$.
Work Step by Step
We want to make a $D^0$ meson.
This has charge = 0, baryon number = 0, strangeness = 0, charm = +1. We assume that bottomness = 0, and topness = 0.
To have charm = 1, one of the quarks must be a c quark. This has a charge of $+\frac{2}{3}e$.
Therefore, since the total charge is 0, the other quark has a charge of $-\frac{2}{3}e$.
The baryon number is 0, so the second quark is an antiquark, in particular, it must be an up antiquark.
$$D^0=c\overline{u}$$
