Among $\mathrm{H}{}_2\mathrm{O}$, $\mathrm{H}{}_2\mathrm{S}$, $\mathrm{H}{}_2\mathrm{Se}$ and $\mathrm{H}{}_2\mathrm{Te}$, the strongest acid in aqueous solution is

The H-E bond enthalpy decreases down the group as the size of E increases, so the hydride loses $\mathrm{H}{}^{+}$ more readily. Acidic character increases $\mathrm{H}{}_2\mathrm{O}<\mathrm{H}{}_2\mathrm{S}<\mathrm{H}{}_2\mathrm{Se}<\mathrm{H}{}_2\mathrm{Te}$, making $\mathrm{H}{}_2\mathrm{Te}$ the strongest acid.