Chemistry: Molecular Approach (4th Edition)

$HF$ is the stronger acid.
The larger the $K_a$, the stronger the acid. This means the acid with larger $K_a$ value is the stronger one. $3.5 \times 10^{-4}$ is greater than $2.9 \times 10^{-8}$. Therefore, $HF$ is stronger than $HClO$.