Answer: The answer to this is option (b) Mg2+
Explanation: The term isoelectronic means "having the same number of electrons". For solving such type of question, first we need the atomic numbers of all the given elements, which also gives us the total number of electrons in these elements. Then based on the ionic valency given, we can add or reduce the number of electrons.
NOTE: The sign + (plus) for valency means that electrons are lost, so subtract it from the atomic number value and the sign - (minus) means extra electrons. So add it to the total number of electrons in the neutral atom (atomic number).
For example, for option (a), we have Calcium as Ca2+. Calcium atomic number being 20, the 2+ valency means that Ca2+ ion is deficit of 2 electrons. Hence the total number of electrons here is 20 - 2 = 18
Calculating it for all other options in the same way,
(b) Mg2+ means Mg = 12, so 12 - 2 = 10
(c) Cl- means Cl = 17 + 1 = 18
(d) S2- means S = 16 + 2 = 18
(e) Ar means Ar = 18
All options other than option (b) have total 18 electrons, so they are ISOELECTRONIC, except Mg2+ which has 10 electrons.