Step 1: Factors governing ionic mobility.
Mobility in water depends on ionic size and extent of hydration; smaller ions hydrate more heavily, slowing their movement.
Step 2: Hydration trend in alkali metals.
Li⁺, the smallest, is most hydrated and thus least mobile; Cs⁺, the largest, is least hydrated and most mobile.
Step 3: Establishing the sequence.
Mobility rises with decreasing hydration: Cs⁺>K⁺>Na⁺>Li⁺.
Step 4: Underlying principle.
Effective hydrated radius, not bare ionic radius, dictates aqueous mobility.
Step 5: Conclusion.
The correct mobility order is Cs⁺>K⁺>Na⁺>Li⁺.