The metal that cannot be obtained by the 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 cation. Thus, among all the given elements, only calcium on electrolysis yields ${\mathrm{H}}_2$ gas at cathode while other yields pure metals at cathode. In case of ${\mathrm{Ca}}^{2+}$ ion, water is reduced in preference to calcium. 

For Cr at cathode, ${\mathrm{Cr}}^{3+}+3{\mathrm{e}}^{-}\to \mathrm{Cr}$

For Ag At cathode, ${\mathrm{Ag}}^{+}+{\mathrm{e}}^{-}\to \mathrm{Ag}$

For Cu At cathode, ${\mathrm{Cu}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Cu}$

For ${\mathrm{Ca}}^{2+}$ At cathode, ${\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to \frac{1}{2}{\mathrm{H}}_2+{\mathrm{OH}}^{-}$