Answer
$MgCl_2$ is a strong electrolyte.
Work Step by Step
As the equation shows, when $MgCl_2$ is dissolved in water, it is divided into $Mg^{2+}$ and $Cl^-$ ions.
Since the arrow only points to the right side, we can assume that $MgCl_2$ is completely transformed into $Mg^{2+}$ and $Cl^{-}$, and this is the main characteristic of a strong electrolyte.