Which one of the following reactions is a DISPROPORTIONATION reaction — i.e., a reaction in which the same element is simultaneously oxidised and reduced?

1. Disproportionation requires ONE element to BOTH gain and lose electrons within the same reaction. 2. Track oxygen in $\mathrm{H}_2\mathrm{O}_2$: it has oxidation number $-1$. In product $\mathrm{H}_2\mathrm{O}$, O is $-2$ (reduced). In $\mathrm{O}_2$, O is $0$ (oxidised). Same element, two directions → disproportionation. So option C is correct. 3. Option A: Na ($0 \rightarrow +1$) and Cl ($0 \rightarrow -1$) are DIFFERENT elements — ordinary redox, not disproportionation. 4. Option B: Zn ($0 \rightarrow +2$) and Cu ($+2 \rightarrow 0$) are different elements — a displacement (single-replacement) reaction. 5. Option D: thermite reaction. Fe and Al change states, but they are different elements — ordinary redox, not disproportionation. _Source: NCERT Class 11 Chemistry Part 2, Ch 7, §7.3.1 "Types of Redox Reactions", p. 9 ¶1 (Disproportionation)._