Reduction decreases oxidation number Steps:

• Identify oxidation numbers of elements in underlined substances before and after reaction.
• Reduction occurs if oxidation number decreases (gain of electrons).
• In choice B, underlined CuO has Cu at +2; in product Cu at 0, so decreased.
• Compare to other choices where underlined substances increase in oxidation number or are not reduced.

Why B is correct:

• Cu in CuO decreases from +2 to 0 in Cu, matching reduction definition (oxidation number decreases).

Why the others are wrong:

• A: Underlined Na increases from 0 to +1 in NaCl (oxidized).
• C: Underlined Zn increases from 0 to +2 in Zn²⁺ (oxidized).
• D: Identical to C; underlined Zn oxidized (not enough information to distinguish).

Final answer: B