The correct option is A Rb+>K+>Na+>Li+
Smaller the size of the ion, more is the hydration and therefore lesser is the mobility.
As we know, on moving down the group in the periodic table, ionic radius increases. So, the order of ionic size will be: Rb+>K+>Na+>Li+ and order of hydration will be : Li+>Na+>K+>Rb+. Hence, the order of mobility will be: Rb+>K+>Na+>Li+