Which one of the following metals cannot be obtained on electrolysis of an aqueous solution of its salts?

The correct option is B Mg

The reduction potential of Mg is less than that of water $E^0=0.83V$. Hence their ions in the aqueous solution cannot be reduced instead water will be reduced. The reaction will occur as follows:

$2H_{2}O+2e^{-}\to H_2+2O{H}^{-}$