The density of alkali metals is in the order

As we down in a group then its size increases and also volume of the atom increases as the number of electrons and protons increases. But the increase in mass is greater than that of increase in volume so the density which is defined as mass per unit volume will increase generally. Density of alkali metals increases down the group except potassium. Therefore, correct order of density of given alkali metals is $K < Na < Rb < Cs$