Cell notation of a galvanic cell is represented as $\mathrm{Mg}\left(\mathrm{s}\right)/{\mathrm{Mg}}^{2+}\left({\mathrm{M}}_1\right)//{\mathrm{Br}}^{-}\left({\mathrm{M}}_2\right)//\frac{1}{2}{\mathrm{Br}}_2\left(1\mathrm{atm}\right), \mathrm{Pt}\left(\mathrm{s}\right).$ Emf of the cell is maximum when the values of M₁ and M₂ are

Anode (LHE) ;  $\mathrm{Mg}\left(\mathrm{s}\right)\stackrel{ }{\to }{\mathrm{Mg}}^{2+}\left({\mathrm{M}}_1\right)+2\overline{\mathrm{e}};$

Cathode (RHE) ;  ${\mathrm{Br}}_2\left(1\mathrm{atm}\right)+2\overline{\mathrm{e}}\stackrel{ }{\to }2{\mathrm{Br}}^{-}\left({\mathrm{M}}_2\right)$

Net cell reaction : $\mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{Br}}_2\left(1\mathrm{atm}\right)\stackrel{ }{\to }{\mathrm{Mg}}^{2+}\left({\mathrm{M}}_1\right)+2{\mathrm{Br}}^{-}\left({\mathrm{M}}_2\right)$

$$\begin{gathered} {\mathrm{E}}_{\mathrm{cell}}={\mathrm{E}}_{\mathrm{cell}}^0-\frac{0.0591}{2}\log \left[{\mathrm{Mg}}^{2+}\right]{\left[{\mathrm{Br}}^{-}\right]}^2 \\ \therefore {\mathrm{E}}_{\mathrm{cell}}\propto -\log \left[{\mathrm{Mg}}^{2+}\right]{\left[{\mathrm{Br}}^{-}\right]}^2 \end{gathered}$$

Thus lesser the product of $\left[{\mathrm{Mg}}^{2+}\right]{\left[{\mathrm{Br}}^{-}\right]}^2$ higher will be the ${\mathrm{E}}_{\mathrm{cell}}$;

The product of  $\left[{\mathrm{Mg}}^{2+}\right]{\left[{\mathrm{Br}}^{-}\right]}^2$in each case is

(1)  …. 1 

(2)  …. 10^-1

(3)  …. 10^-2

(4) …. 10^-5

Maximum emf of the cell is when $\left[{\mathrm{Mg}}^{2+}\right]$=0.1 M and $\left[{\mathrm{Br}}^{-}\right]$ =0.01 M