Final answer:
When a Cl- ion forms, it gains an electron, which means it undergoes reduction.
Step-by-step explanation:
Oxidation is the loss of electrons, gain of oxygen, or loss of hydrogen, while reduction is the opposite, i.e., gain of electrons, loss of oxygen, or gain of hydrogen. When a Cl- ion forms, it gains an electron, which means it undergoes reduction. Reduction is defined as the gaining of electrons and the reduction of charge.