The metal that cannot be obtained by electrolysis of an aqueous solution of its salts is

Higher the position of element in the electrochemical series more difficult is the reduction of its cations. If Ca2+ (aq) is electrolysed, water is reduced in preference to it. Hence, it cannot be reduced electrolytically from their aqueous solution.Ca2+(aq)+H2O⟶Ca2++OH−+H2↑