The answer is a. Rubidium
A cation is an ion that has a positive charge. The charge positive because it loses electrons. The radius of an atom is influenced by the electron because it is found at the outer layer of the atom. Since cation has less electron and anion get more electron, the radius of cation will be smaller than anion.
Rubidium is the biggest cation because it is located in the lowest row, thus having most electron count.