Lemon juice is used to slow apple oxidation when macerating apple pie filling.
Cut apples turn brown because enzymes react with oxygen after the fruit’s cells are damaged. Lemon juice lowers the mixture’s pH and supplies citric acid, helping slow that browning while the slices sit with sugar and other filling ingredients. It also contributes a bright tart note that balances sweetness.
“Macerating” means allowing fruit to stand with sugar or another ingredient so it releases juice and absorbs flavor. In apple pie preparation, that liquid becomes part of the filling, while a thickener such as cornstarch may help control the released juices during baking.
Lemon juice is not present only for color protection. It is also a familiar flavoring in apple pie, alongside cinnamon and nutmeg. Other acidic liquids can sometimes be substituted in recipes, but the stored answer is the canonical ingredient named by Wikipedia for slowing oxidation during maceration. Lime juice, vinegar, and orange juice are therefore not the expected answer here.