JEE Main · 2024 · 01 Feb 2024 · Shift 1 · Redox Reactions and Electrochemistry

A disproportionation reaction is one where the same element is simultaneously oxidised and reduced (present in one oxidation state on the reactant side and in two different oxidation states — one higher, one lower — among the products).
(A) ${\text{Cu}}^{+}\to {\text{Cu}}^{2+}+\text{Cu}$: Cu(+1) goes to Cu(+2) and Cu(0) — disproportionation.
(B) $3{\text{MnO}}_4^{2-}+4{\text{H}}^{+}\to 2{\text{MnO}}_4^{-}+{\text{MnO}}_2+2{\text{H}}_2\text{O}$: Mn(+6) goes to Mn(+7) and Mn(+4) — disproportionation.
(C) $2{\text{KMnO}}_4\to {\text{K}}_2{\text{MnO}}_4+{\text{MnO}}_2+{\text{O}}_2$: Mn(+7) goes to Mn(+6) and Mn(+4) (disproportionation of Mn), but oxygen also changes from $-2$ to $0$ — this is not a simple, single-element disproportionation intended here and is excluded by the key.
(D) $2{\text{MnO}}_4^{-}+3{\text{Mn}}^{2+}+2{\text{H}}_2\text{O}\to 5{\text{MnO}}_2+4{\text{H}}^{+}$: Mn(+7) and Mn(+2) both converge to Mn(+4) — this is comproportionation, the reverse of disproportionation, not disproportionation.
So only (A) and (B) are disproportionation reactions.