Final answer:
The pictures of trees in the article visually depict that fall is the most colorful season, showing the transformation of green leaves to vibrant colors due to reduced chlorophyll production in response to shorter days and cooler temperatures.
Step-by-step explanation:
The pictures of the trees help in understanding the main idea of the article because they visually illustrate the biological process described. The images would support the main idea that the color of leaves changes in autumn due to environmental factors such as shorter days and cooler temperatures, which lead to a decrease in chlorophyll production. This allows the other pigments in the leaves, predominantly oranges and yellows, to become visible, contributing to the characteristic 'fall colors'.
Given the provided options and the context from the article, it seems that the pictures most likely support the idea that fall is the most colorful season of the year (D). This is because the main idea presented is how the leaves change from green to glorious colors in the fall as a biological response to changes in the environment.