Which one of the following ion has the highest value of ionic radius ?

No. of electrons in O²⁻ = 10
No. of electrons in B³⁺ = 8
No. of electrons in Li⁺ = 4
No. of electrons in F⁻ =  10

This can be explained on the basis of z/e{nuclear charge /no. of electrons}, as z/e ratio increases, the size decreases and when z/e ratio decreases the size increases.
For Li⁺,z/e = 3/2 =1.5
For B³⁺,z/e = 5/2 =2.5
For O²⁻,z/e = 8/10 =0.8
For F⁻, z/e = 9 /10=0.9
Hence, O²⁻ has highest value of ionic radius.