The correct answer is D. They stop making chlorophyll
Step-by-step explanation:
The regular green color in leaves is mainly the result of chlorophyll, which is essential for photosynthesis. This substance stops being produced by plants during the fall, and this causes leaves to lose their characteristic color. Additionally, this process is the result of shorter days, and therefore fewer daylight hours, which reduces or stops the production of chlorophyll. At the same time, the carotenoids which are red, yellow, and orange pigments begin to be visible. Thus, the reason leaves change color in the fall is that they stop making chlorophyll.