Answer
$HF$ is stronger.
Work Step by Step
- Since $O$ and $F$ are on the same row of the periodic table, we have to compare their electronegativity. The acid with the more electronegative element will be the more acidic.
- The electronegativity increases from the left to the right of the row.
- $F$ is closer to the left side.
So, the $H - F$ bond is weaker than the $H - O$ bonds, making $HF$ the stronger acid.
