Answer: Option (d) is the correct answer.
Step-by-step explanation:
The given elements are Rb, Na, K and Li and all these elements are group 1 elements.
These are also known as alkali metals. These elements are placed in the order from top to bottom as Li, Na, K, Rb.
It is known that when we move down a group then there is increase in the size of atom. As a result, atomic radius also increases.
Thus, atom at the top which is lithium (Li) will have the smallest atomic radius.