Redox Reactions

Which of the following reactions are disproportionation reactions?

(a) $2\mathrm{C}{\mathrm{u}}^{+}\to \mathrm{C}{\mathrm{u}}^{2+}+\mathrm{C}{\mathrm{u}}^0$

(b) $3\mathrm{Mn}{\mathrm{O}}_4^{2-}+4{\mathrm{H}}^{+}\to 2Mn{\mathrm{O}}_4^{-}+\mathrm{Mn}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}$

(c) $2\mathrm{KMn}{\mathrm{O}}_4\to {\mathrm{K}}_2\mathrm{Mn}{\mathrm{O}}_4+\mathrm{Mn}{\mathrm{O}}_2+{\mathrm{O}}_2$

(d) $2\mathrm{Mn}{\mathrm{O}}_4^{-}+3M{\mathrm{n}}^{2+}+2{\mathrm{H}}_2\mathrm{O}\to 5Mn{\mathrm{O}}_2+4{\mathrm{H}}^{+}$

Select the correct option: