Increasing order of atomic radii
Li, Na, K, Rb, Cs :

The correct option is A Li < Na < K < Rb < Cs
Concept: As we move from top to bottom in group the atomic radius will be increased, because the in coming electron enters into a new sub shell.
Li, Na, K, Rb, Cs are IA group elements from Li to Rb the atomic radius will be increased. So the correct order of atomic radius is Li < Na < K < Rb < Cs
Hence option A is correct.