Question

solubility of NaF in water is less than that of RbF

Answer 1

When we see along a group solubility increases therefore Na<Rb
=> solubility  of NaF < RbF

Answer 2

According to the periodic trend of the first group, the solubility of an element in water increases as we go down the group.
However it can be noted that the reaction of these pure elements in water get more explosive and destructive as we go down the group.(There are some interesting YouTube videos that will show you this!)