The ionic conductance of the following cations in a given concentration are in the order

In a given concentration means aqueous solution.In aqueous solution Size of hydrated ions depends on degree of Hydration which inturn depends on size of cation.Degree of Hydration ∝1/size of ionSize of Hydrated ion ∝degree of HydrationLarger the size of hydrated ion lesser is the ionic mobility and lesser is the ionic conductance.