consider the isoelectronic species Na+, Mg2+, F-, and O2- The correct order of increasing length of their radii is

mg2+

 

 

@ gokul 
they all are isoelectronic species that means they all contain same no of electrons ,, 
they differ by the no of protons , the ion having the highest electron/ proton ration will be the largest in this series . 
so, by keeing that in mind , u have to arrenge it . 
the below sequence are correct . 
Mg2+  
HOPE IT CLEARS YOUR DOUBT 
ALL THE BEST ..