Final answer:
The substance found in the highest concentrations in the Earth's atmosphere is oxygen, making up about 21% of the atmosphere.
Step-by-step explanation:
The correct answer is D. oxygen. The atmosphere at Earth's surface is composed of roughly 78% nitrogen (N₂), 21% oxygen (O₂), and 1% argon (Ar), with traces of water vapor (H₂O), carbon dioxide (CO₂), and other gases. While water vapor, carbon dioxide, and argon are present, they exist in much smaller amounts than oxygen. Specifically, oxygen has a partial pressure much greater than that of carbon dioxide or any other gases aside from nitrogen. Therefore, amongst the options provided, oxygen is the most abundant element in the atmosphere.