Which of the following will increase the voltage of the cell, 

$\mathrm{Cu}+2{\mathrm{Ag}}^{+}⟶{\mathrm{Cu}}^{+2}+2\mathrm{Ag}$ 

Cell voltage will increase, either by decreasing the concentration of Cu²⁺  ion or by increasing the concentration of Ag⁺.

The Nernst equation is given as 

$\mathrm{EMF}\hspace{0.17em}= {\mathrm{E}}_{\mathrm{cell}}^{\mathrm{o}}-\frac{0.0591}{\mathrm{n}}\log {\mathrm{Q}}_{\mathrm{c}}$

${\mathrm{Q}}_{\mathrm{c}}=\frac{\left[{\mathrm{Cu}}^{2+}\right]}{\left[{\mathrm{Ag}}^{+}\right]}$

Lesser is the value of Q_c, greater is the cell voltage.

As Ag⁺ concentration increases , Q_c value decreases and EMF increases.