When you are asked this question, you are looking for the one species that does not have the same electron configuration as the others. In this case,
Start with Ar and look at where it is on the periodic table.
Next, you will notice that if you take 2 electrons away from Ca, it will land you on the same spot as Ar on the periodic table.
Next, if you add an electron to Cl, it will have the same configuration as the element directly to its right on the table, that being Ar.
Next, if you add 2 electrons to S, the configuration will be like that of Ar.
Lastly, taking an electron away from Na will give it the same configuration as Ne, not Ar.
Thus, the only species here that differs in terms of configuration is Na+.
