:
Option (c) is the correct answer.
Step-by-step explanation:
It is known that when we move down a group then there is increase in size of atom. This is because there occurs increase in number of shells in the atom.
Also, when an atom loses electrons then it acquires a positive charge and size of the atom decreases. On the other hand, when an atom gain electrons then it acquires a negative charge and size of the atom increases.
Since, both Rb and K are same group elements and Rb has larger atomic size than K.
Hence, we can conclude that out of the given options K^{+}
+ has the smallest size.