Which of the following reactions is used to make a fuel cell?

Options

1. A$$\begin{aligned} & \mathrm{Cd}_{(s)}+2 \mathrm{Ni}(\mathrm{OH})_{3(s)} \\ & \rightarrow \mathrm{CdO}_{(s)}+2 \mathrm{Ni}(\mathrm{OH})_{2(s)}+\mathrm{H}_2 \mathrm{O}_{(l)}\end{aligned}$$
2. B$\mathrm{Pb}_{(s)}+\mathrm{PbO}_{2(s)}+2 \mathrm{H}_2 \mathrm{SO}_{4(a q)}$ $\rightarrow 2 \mathrm{PbSO}_{4(s)}+2 \mathrm{H}_2 \mathrm{O}_{(l)}$
3. C$2 \mathrm{H}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})} \rightarrow 2 \mathrm{H}_2 \mathrm{O}(l)$
4. D$2 \mathrm{Fe}_{(s)}+\mathrm{O}_{2(g)}+4 \mathrm{H}^{+}{ }_{(a q)} \rightarrow 2 \mathrm{Fe}^{2+}{ }_{(a q)}+2 \mathrm{H}_2 \mathrm{O}_{(l)}$