Difference between ionic and atomic radius (stuck on problem)?

atomic radius is the radius of the neutral atom of an element. ionic radius is the radius of a charged version of the element. If you add an electron onto an atom, the radius increases because more electrons take more space, so removing an electron decreases the radius for the same reason. So what you want to look for in this question is which element loses an electron when it becomes ion. In other words, you want to find the element that forms a cation. From this list, the only element that forms a cation is sodium, so that is the answer