Answer
$HCl$ is stronger.
Work Step by Step
- Since $Cl$ and $F$ are on the same column of the periodic table, we have to compare the bond strength. The one with the weaker bond will be the more acidic.
- The bond strength increases from the bottom to the top of the column, therefore, the one closer to the bottom have the weaker bond.
- $Cl$ is closer to the bottom.
So, the $H - Cl$ bond is weaker than the $H - F$ bond, making $HCl$ the stronger acid.
