Rank these ions according to ionic radius.
rb+
sr2+
as3-
se2-
br-

Atomic radius increases going down and decreases left to right. Look at where these are located on the periodic table and apply this.

The correct answer is c b c a d

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.