BITSAT 2019 Slot 2 Solved Paper

Key idea Mobility of alkali ions in aqueous solution ∝ size of ion
∝

1

size of hydrated ion

The mobility of alkali ions is inversely proportional to the size of ion in hydrated state, because smaller the ion more it will get hydrated and becomes larger in size, thus its ionic mobility decreases.
Hence, correct order of mobility is
Rb+>K+>Na+>Li+