Reduction decreases oxidation state Steps:

• Define reduction as a decrease in oxidation number (gain of electrons).
• Examine oxidation states in each reaction or process.
• Identify if any element's oxidation state decreases.
• Select the option showing clear reduction.

Why D is correct:

• In sand (SiO₂), silicon is +4; in silicon (Si), it is 0, so Si gains electrons (reduced) per the definition of reduction.

Why the others are wrong:

• A: CaCO₃ decomposes to CaO + CO₂; Ca (+2), C (+4), and O (-2) oxidation states unchanged—no reduction.
• B: Copper to brass is alloy formation (Cu + Zn)—physical mixture, no redox change.
• C: Ethene to poly(ethene) is addition polymerization forming C-C bonds—oxidation states of C (-2 in both) unchanged.

Final answer: D