Which of these is an isoelectronic series?A) Na+, K+, Rb+, Cs+B) K+, Ca2+, Ar, S2–C) Na+, Mg2+, S2–, Cl–D) Li, Be, B, CE) none of these (A-D)

The term "isoelectronic" refers to different species (atoms, ions, or molecules) that have the same number of electrons.

Let's examine each option:

A) Na+ (10 electrons), K+ (18 electrons), Rb+ (36 electrons), Cs+ (54 electrons) - They do not have the same number of electrons.

B) K+ (18 electrons), Ca2+ (18 electrons), Ar (18 electrons), S2– (18 electrons) - All of these species have the same number of electrons, which is 18.

C) Na+ (10 electrons), Mg2+ (10 electrons), S2– (18 electrons), Cl– (18 electrons) - They do not have the same number of electrons.

D) Li (3 electrons), Be (4 electrons), B (5 electrons), C (6 electrons) - They do not have the same number of electrons.

E) None of these (A-D) - This is not the correct answer because option B is an isoelectronic series.

So, the answer is B) K+, Ca2+, Ar, S2–. These species all have 18 electrons, making them isoelectronic.