The correct answer is D. 21%.
Oxygen is the most abundant element in the Earth’s atmosphere, making up about 21% of the air we breathe. It is essential for life, as it is used in cellular respiration to produce energy. Oxygen is also used in many industrial processes, such as the production of steel and chemicals.
The other options are incorrect. Option A, 44%, is the percentage of nitrogen in the Earth’s atmosphere. Option B, 23%, is the percentage of argon in the Earth’s atmosphere. Option C, 40%, is the percentage of carbon dioxide in the Earth’s atmosphere.