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The correct order of the mobility of the alkali metal ions in aqueous solution is :
A
$Li^+ \gt Na^+ \gt K^+ \gt Rb^+$
B
$Na^+ \gt K^+ \gt Rb^+ \gt Li^+$
C
$K^+ \gt Rb^+ \gt Na^+ \gt Li^+$
D
$Rb^+ \gt K^+ \gt Na^+ \gt Li^+$
Detailed Solution
The extent of hydration of an ion is proportional to its charge density.
$Li^+$ is the smallest alkali metal ion, so it has the highest charge density and is the most heavily hydrated.
Down the group the ionic size increases and the degree of hydration decreases: $Li^+ \gt Na^+ \gt K^+ \gt Rb^+$
Therefore the hydrated size is largest for $Li^+$ and smallest for $Rb^+$.
A bigger hydrated ion moves more slowly through the solution, so $Li^+(aq)$ has the lowest mobility.
Order of mobility: $Rb^+ \gt K^+ \gt Na^+ \gt Li^+$
$Li^+$ is the smallest alkali metal ion, so it has the highest charge density and is the most heavily hydrated.
Down the group the ionic size increases and the degree of hydration decreases: $Li^+ \gt Na^+ \gt K^+ \gt Rb^+$
Therefore the hydrated size is largest for $Li^+$ and smallest for $Rb^+$.
A bigger hydrated ion moves more slowly through the solution, so $Li^+(aq)$ has the lowest mobility.
Order of mobility: $Rb^+ \gt K^+ \gt Na^+ \gt Li^+$