Reduction involves decrease in oxidation number

Steps:

• Recall reduction: gain of electrons or drop in oxidation state of the underlined reactant.
• For each option, assign oxidation numbers to the underlined substance before/after reaction.
• Check if oxidation number decreases (reduced) or increases (oxidized).
• Identify the option where underlined substance's oxidation number decreases.

Why B is correct:

• In Mg(s) + CuO(s) → MgO(s) + Cu(s), underlined CuO has Cu at +2; in Cu(s), it's 0—oxidation number decreases, so reduced.

Why the others are wrong:

• A: Underlined Na(l) at 0 becomes Na+ at +1 in NaCl—increases, oxidized.
• C: Underlined Zn(s) at 0 becomes Zn²⁺ at +2—increases, oxidized.
• D: Underlined Fe(s) at 0 becomes Fe²⁺ at +2—increases, oxidized.

Final answer: B