When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue. When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colorless precursors to brown-colored secondary products.
I believe It's 3. If it isn't it's probably 4, because that also has to do with change. (If I'm being honest none of these really pertain to oxidation, since it has to do with weathering things but eh.)
