Final answer:
In Biology, CO₂ concentrations in the atmosphere show a natural fluctuation each year during spring and autumn.
Step-by-step explanation:
The correct answer is B) Spring and autumn. Records of CO₂ concentrations in the atmosphere show a natural fluctuation each year during spring and autumn. This fluctuation is caused by the seasonal cycle of photosynthesis by plants. During spring and autumn, plants grow and absorb CO₂, leading to a decrease in atmospheric CO₂ concentration. Conversely, during summer and winter, plant growth is limited, resulting in an increase in CO₂ concentration.