1. Concept of disproportionation

A disproportionation reaction is one in which the same species undergoes both:

• oxidation (increase in oxidation number), and
• reduction (decrease in oxidation number).

So, we check whether the same element in the same reactant ends up in two different oxidation states in the products.

2. Option A: $2\mathrm{H}_2\mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}+\mathrm{O}_2$

Find oxidation state of oxygen:

• In $\mathrm{H}_2\mathrm{O}_2$, oxygen is $-1$
• In $\mathrm{H}_2\mathrm{O}$, oxygen is $-2$
• In $\mathrm{O}_2$, oxygen is $0$

Thus, oxygen from $-1$ goes to:

• $-2$ (reduction)
• $0$ (oxidation)

Hence, this is a disproportionation reaction.

3. Option B: $2\mathrm{NO}_2+\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{HNO}_3+\mathrm{HNO}_2$

Oxidation state of nitrogen:

• In $\mathrm{NO}_2$: let oxidation state of N be $x$ $x+2(-2)=0 \Rightarrow x=+4$
• In $\mathrm{HNO}_3$: N is $+5$
• In $\mathrm{HNO}_2$: N is $+3$

Thus, nitrogen from $+4$ goes to:

• $+5$ (oxidation)
• $+3$ (reduction)

So, this is also a disproportionation reaction.

4. Option C: $\mathrm{MnO}_4^-+4\mathrm{H}^+ +3\mathrm{e}^- \rightarrow \mathrm{MnO}_2+2\mathrm{H}_2\mathrm{O}$

Oxidation state of Mn:

• In $\mathrm{MnO}_4^-$: $x+4(-2)=-1 \Rightarrow x=+7$
• In $\mathrm{MnO}_2$: $x+2(-2)=0 \Rightarrow x=+4$

Mn changes from $+7$ to $+4$ only. This is only reduction, not simultaneous oxidation and reduction of the same species.

Hence, this is not a disproportionation reaction.

5. Option D: $3\mathrm{MnO}_4^{2-}+4\mathrm{H}^+ \rightarrow 2\mathrm{MnO}_4^-+\mathrm{MnO}_2+2\mathrm{H}_2\mathrm{O}$

Oxidation state of Mn:

• In $\mathrm{MnO}_4^{2-}$: $x+4(-2)=-2 \Rightarrow x=+6$
• In $\mathrm{MnO}_4^-$: Mn is $+7$
• In $\mathrm{MnO}_2$: Mn is $+4$

Thus, Mn from $+6$ goes to:

• $+7$ (oxidation)
• $+4$ (reduction)

So, this is a disproportionation reaction.

6. Final conclusion

The reaction which is not an example of disproportionation is: $\boxed{\text{C}}$

7. Comparison with stored correct answer

Stored correct answer: C

My derived answer: C

They match.