Final answer:
Leaves are red in the fall because the chlorophyll breaks down with less sunlight, allowing the red pigments like anthocyanins to become visible.
Step-by-step explanation:
Leaves turn red in the fall primarily because the chlorophyll, the green pigment responsible for photosynthesis, breaks down due to shorter days and cooler temperatures. As chlorophyll diminishes, it allows the red, orange, and yellow pigments, such as carotenoids and anthocyanins, which are normally present but masked by the strong green of the chlorophyll, to become visible. The red color in some leaves is mainly due to the presence of anthocyanins.