Answer:- [tex]Se^2^-[/tex]
Explanations:- Atomic number of Se is 34 and it forms [tex]Se^2^-[/tex] after accepting two electrons and there becomes total 36 electrons to it.
Atomic number of Sr is 38 and it forms [tex]Sr^2^+[/tex] on losing two electrons from it's valence shell and there also becomes total 36 electrons to it.
Now, both of these are isoelectronic means having with equal number of electrons but the cation(positively charged ion) has more protons as compared to the anion(negatively charged ion) and because of this it has more effective nuclear charge and it's size is smaller.
The anion has less effective nuclear charge and so it's size is bigger.
Hence, the right choice is [tex]Se^2^-[/tex] .
