Answer
$Ar$ has the same number of electrons and the same electron configuration as $Cl^-$.
Work Step by Step
$Cl^{-}$
A neutral $Cl$ atom has 17 electrons. Adding 1 electron to become $Cl^{-}$ cation, it now has 18 electrons. The neutral atom that has 18 electrons is $Ar$.
The electron configuration of $Cl$ is $[Ne]3s^23p^5$. To become $Cl^-$, 1 electron must be added. The 3p-orbital are now only occupied by 5 electrons, so 1 electron would be willingly added to 3p-orbital so that it would reach the ultimate stability of all spaces filled, which is 6 electrons for a p-orbital.
Then, the electron configuration of $Cl^-$ is $[Ne]3s^23p^6$. That is also the electron configuration of $Ar$.
