Which of the following best classifies as a COMBINATION redox reaction (i.e. two or more substances combine to form a single product, with electron transfer)?

1. A COMBINATION reaction has the form $A + B \rightarrow C$. To also be redox it must involve electron transfer. 2. Option C: $2\mathrm{Mg} + \mathrm{O}_2 \rightarrow 2\mathrm{MgO}$. Two reactants, one product. Mg goes $0 \rightarrow +2$ (oxidation); O goes $0 \rightarrow -2$ (reduction). ✓ Combination redox. 3. Option A is the REVERSE shape ($C \rightarrow A + B$) — a decomposition. 4. Option B is a displacement reaction: Zn DISPLACES Cu from CuSO$_4$. Not combination. 5. Option D is disproportionation of $\mathrm{H}_2\mathrm{O}_2$ (same element oxidised and reduced). _Source: NCERT Class 11 Chemistry Part 2, Ch 7, §7.3.1 "Types of Redox Reactions — Combination", p. 7._