The correct option is A
Cl−, Ar, K+
Isoelectronic species are those species that have the same number of electrons.
a) Cl−,Ar,K+(Cl−=17+1,Ar=18,K+=19−1)
b) Ne,F+,O2−(Ne=10,F+=9−1,O2−=8+2)
c) N3−,O2−,Ar(N3−=7+3,O2−=8+2,Ar=18)
d) Ar,Na+,Mg2+(Ar=18,Na+=11−1,Mg2+=12−2)
All the elements in a) have the same number of electrons ie., 18