Answer
$F^-$ is the stronger base.
Work Step by Step
$F^-$ is the stronger base because it readily accepts the $H^+$ ion to form $HF $, which is a weak acid and does not easily ionize back to $H^+$ and $F^-$. Whereas, $Cl^-$ is a weaker base because after accepting a $H^+$ ion it readily ionizes into $H^+$ and $Cl^-$ .
