Which of the following has the smallest radius?

A

P

B

S

C

Cl

D

Cl−

E

S2−

Dear student 

As you go across a period, electrons are added to the same energy level.  At the same time, protons are being added to the nucleus.  The concentration of more protons in the nucleus creates a higher effective nuclear charge.  In other words, there is a stronger force of attraction pulling the electrons closer to the nucleus resulting in a smaller atomic radius.Also, Anions are larger than their respective atoms as electron-electron repulsion forces them to spread further apart. 
Hence, Cl has the smallest radius.