Answer
$HI$ is more acidic.
Work Step by Step
Let's compare the conjugate bases:
$HBr: Br^-$
$HI: I^-$
Since they are in the same column, we need to compare their sizes, to know which one is more stable.
- $I$ is bigger than $Br$, so $I^-$ has more space to stabilize the charge.
Therefore, $HI$ is more acidic.
