For the discharge of equal masses of the following ions, the number of electrons required is maximum in the case of

According o Faraday's law of electrolysis 

Mass of substance deposited(w) =$\frac{\mathrm{E} \mathrm{X}\hspace{0.17em}\mathrm{i} \hspace{0.17em}\mathrm{X} \mathrm{t} }{96500}$$=\frac{\mathrm{M} \mathrm{X}\hspace{0.17em}\mathrm{i} \hspace{0.17em}\mathrm{X}\hspace{0.17em}\mathrm{t} }{\mathrm{n} \mathrm{X}\hspace{0.17em}\mathrm{t} }$

E= equivalent weight 

i= current 

t= time 

M= molecular mass 

n= number of electron required . 

The equation indicates us 

The number of electrons required $\alpha$ number of gram equivalents 

number of gram equivalents = $\frac{\mathrm{weight} }{\mathrm{Equivalent} \mathrm{weight}}$

The substance which has less equivalent weight will be deposited more equivalents for the same mass and required electrons are more. 

E_H=1

E_cu=32

E_Ag = 108

E_Al = 9

As the equivalnet weight of H⁺ is less it required more number of electrons.