Final answer:
Water vapor is the most variable atmospheric component in concentration, ranging from nearly zero up to 4%, due to processes such as evaporation and condensation.
Step-by-step explanation:
Among the atmospheric components listed, water vapor is the most variable in its concentration. Nitrogen, oxygen, and carbon dioxide are present in fairly consistent proportions, with nitrogen comprising approximately 78%, oxygen about 21%, and carbon dioxide about 0.04% of the Earth's atmosphere at sea level. However, the concentration of water vapor can range from nearly zero up to 4%, depending on various factors such as temperature and humidity. This high variability is due to the constant exchange of water between the Earth's surface and the atmosphere through evaporation and condensation processes.