Answer:
B. Electrons are gained, so the oxidation number decreases.
Step-by-step explanation:
Reduction is the gain of electrons.
Oxidation number is the charge that an atom appears to have when we count its electrons in a specific way.
Electrons have a negative charge so, if an atom gains electrons, its charge (oxidation number) becomes more negative. The oxidation number decreases.
If electrons are given off, the atom is being oxidized. Loss of electrons is oxidation.