The ionic mobility of H+ in water (lambda deg approx 350 S cm^2 mol^-1) is far higher than that of any other cation. The main reason is that

charge is transferred by proton hopping along hydrogen-bonded water molecules, so the whole hydrated ion need not move

H+ has the smallest ionic radius and is therefore the least hydrated cation

H+ has the highest charge density and is attracted most strongly towards the cathode

H3O+ moves through water as a bare proton without any hydration shell

JEE Main levelSingle correct optionDifficulty 2 of 3ec218

The ionic mobility of $H X^{+}$ in water ( $λ^{\circ} \approx 350 S c m^{2} mo l^{- 1}$ ) is far higher than that of any other cation. The main reason is that

Acharge is transferred by proton hopping along hydrogen-bonded water molecules, so the whole hydrated ion need not move
B $H X^{+}$ has the smallest ionic radius and is therefore the least hydrated cation
C $H X^{+}$ has the highest charge density and is attracted most strongly towards the cathode
D $H X_{3} O X^{+}$ moves through water as a bare proton without any hydration shell

In water the proton exists as

H X_{3} O X^{+}

and is strongly hydrated; if it moved like a normal ion its mobility would be similar to

Na X^{+}

K X^{+}

. The anomalously high value arises from the Grotthuss mechanism: a proton on

H X_{3} O X^{+}

jumps to a neighbouring water molecule through a hydrogen bond, which then passes a proton on, so charge moves much faster than any single ion.

OH X^{-}

shows a similar but smaller effect. The force on an ion in a field depends on its charge, not its charge density, so (C) is wrong.