Which of the following isoelectronic species has the largest radius?

Solution:

Atomic radius decreases as we move from left to right in a period and it increases when we go down a group.

They all are isoelectronic they all contain the same no. of electrons 

for isoelectronic ion $\mathrm{ionic} \mathrm{radii} \mathrm{\alpha }\frac{1}{\mathrm{No}. \mathrm{of} \mathrm{protons} }$

Therefore, isoelectronic species has the largest radius is