Ionic mobility of which of the following alkali metal ions is lowest when aqueous solution of their salts is put under an electric field? (NEET 2017)

The ionic mobility of alkali metals in the aqueous solution with the application of an electric field is directly proportional to the atomic size. The atomic size increases down the group and it decreases from left to right. Therefore, the Lithium has the lowest ionic mobility as it has a small atomic size or large hydrated ion size.