Answer
$D_S^+=c\overline{s}$.
Work Step by Step
We want to make a $D_S^+$ meson.
This has charge = 1, baryon number = 0, strangeness = 1, 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 1, the other quark has a charge of $\frac{1}{3}e$.
The baryon number is 0, so the second quark is an antiquark. SInce this meson has strangeness of +1, the other quark must be a strange antiquark.
$$D_S^+=c\overline{s}$$
