Reduction decreases oxidation number

Steps:

• Recall reduction means gain of electrons or decrease in oxidation state of an element.
• Examine each option for changes in oxidation states.
• Identify redox reactions by tracking element oxidation numbers.
• Select the option where an element's oxidation state decreases.

Why D is correct:

• In sand (SiO₂), silicon has +4 oxidation state; in elemental silicon, it is 0, a decrease showing reduction (e.g., via carbothermic reduction: SiO₂ + 2C → Si + 2CO).

Why the others are wrong:

• A: CaCO₃ → CaO + CO₂ is thermal decomposition; Ca and O oxidation states unchanged (+2 and -2).
• B: Copper to brass forms an alloy by mixing Cu and Zn; no electron transfer or oxidation state change.
• C: Ethene to poly(ethene) is addition polymerization; carbon oxidation states remain -2.

Final answer: D