To determine which alkali metal ion has the lowest ionic mobility in an aqueous solution, we need to consider the size of the hydrated ions.$\\$Ionic mobility is inversely related to the size of the hydrated ion. Smaller hydrated ions move more slowly through the solution.$\\$The alkali metal ions in question are:$\\$• Lithium (Li$^+) \\$• Sodium (Na$^+) \\$• Potassium (K$^+) \\$• Rubidium (Rb$^+) \\$When these ions are in aqueous solution, they become hydrated, forming a shell of water molecules around them.$\\$The size of the hydrated ion depends on the charge density of the ion. Smaller ions with higher charge density attract more water molecules, leading to larger hydrated sizes.$\\$Among the alkali metals, lithium ion $({Li}^+)$ is the smallest and has the highest charge density, resulting in the largest hydrated size.$\\$Therefore, Li$^+$ has the lowest ionic mobility in an aqueous solution because its large hydrated size makes it move more slowly under an electric field.$\\$Thus, the correct answer is Option 3: Li.