Which is the largest ion in the isoelectronic series S2-, Cl-, K+, Ca2+?

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15. The following ions comprise an isoelectronic series: S2, Cl, K+, and Ca2+
Which is the largest ion?

A. S2-
B. Cl-
C. K+
D. Ca2+
E. All should be the same size.

Don't even know what one is.
 
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Does the term "isoelectronic series" do anything for this problem?
 
It just means that all ions have exactly the same number of electrons, the only thing that changes is nucleus charge.
 
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It quite self-explanatory, atoms with the same number of electrons. You should be able to figure out the electron number in a neutral atom of the element, and further deduce the electron number of a corresponding ion.

In reference to the size of each of these, you'll need to consider the relative proton number with the electron number, effective nuclear charge. Typically, a isoelectronic cation is smaller than the anion because you've got the same number of electrons for both with a smaller proton number for the latter.
 
Not to blow any trumpets here but this essentially summarizes what has been said above by Borek and GenChemTutor, about comparing radii among isoelectronic ions.