The correct statement is:
Carbon dioxide in the atmosphere would be highest in early spring and lowest in the fall because photosynthesis takes carbon dioxide out of the atmosphere.
This is true because, in the Northern Hemisphere, vegetation such as trees and plants is actively photosynthesizing during the growing season (spring and summer), which removes carbon dioxide from the atmosphere. In the fall, as plants lose their leaves and photosynthesis decreases, carbon dioxide levels rise. By early spring, there has been significant decomposition and breakdown of organic matter during the winter, leading to higher atmospheric carbon dioxide levels until the new growing season begins.