The correct decreasing order of atomic size among the following species is :   Ar, K+, Cl, S2-, Ca2+
(a) Ca2+ > K+ > Ar  > Cl > S2-
(b) K+ > Ca2+ > Cl> Ar > S2-
(c) S2- > Cl > Ar  > K+ > Ca2+
(d) S2- >Ar > Cl > Ca2+ > K+

These are isoelectronic species which means all have same number of electrons.
So, now just remember this,
Number of negative charge is directly proportional to Ionic Radii
And Number of + charge is inversely proportional to the ionic radii.
So, combining above two we can say that for isoelectronic species, ionic radii is decreasing from more -ve charge to zero and further to +ve charge.

Hence. Option C will be our correct answer.

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