Study the reaction.
Mg + Cl₂ → Mg2+ + 2Cl¯
What is the oxidizing agent?
Cl2, because it gains electrons and is reduced to Cl- in the
reaction
Mg2+, because it has lost electrons and is oxidized from Mg
Mg, because it loses electrons and is oxidized to Mg2+ in the
reaction
Cr, because it has gained electrons and is reduced from Cl₂