When a slice of fresh apple turns a brown color, this is most likely the result of

This is most likely the result of OXIDATION. When an apple is cut open, the cells of the apple are exposed to the atmosphere, the in built defense mechanism inside the apple that is supervised by certain enzymes react to this situation by reacting with oxygen and creating an oxidized brown layer which protects the apple from foreign bodies.

Where is the picture at

1. aspect is nothing really because the answer is right in front of your face