The correct order of conductivity of ions in water is:

The conductivity (or ionic mobility) of alkali metal cations in water depends on their hydrated radius, not their bare ionic radius. In water, smaller ions have a higher charge density, which attracts more water molecules, forming a larger hydration shell. This larger hydrated ion moves more slowly through the solution, resulting in lower conductivity.

Among the alkali metal cations, the bare ionic radius increases as: $$Li^+ < Na^+ < K^+ < Rb^+ < Cs^+$$. However, the hydrated radius decreases in the same order because smaller bare ions attract more water molecules: hydrated radius of $$Li^+ > Na^+ > K^+ > Rb^+ > Cs^+$$.

Since a smaller hydrated radius means the ion can move faster through solution, the ionic conductivity increases as: $$Li^+ < Na^+ < K^+ < Rb^+ < Cs^+$$, or equivalently $$Cs^+ > Rb^+ > K^+ > Na^+$$.

This matches option (B): $$Cs^+ > Rb^+ > K^+ > Na^+$$.