Answer
$S$ and $O$ are on the same group of the periodic table, therefore, the one closest to the bottom will have the stronger binary acid.
Work Step by Step
This happens because, the bond between the atom and the hydrogen on a binary acid tends to get weaker as the atom get bigger.
Since $S$ is bigger than $O$: $H_2S$ will be stronger.
** This is only true if we are comparing elements from the same group of the periodic table.
