What is the constant level of oxygen found in the atmosphere?

The constant level of oxygen found in the atmosphere is approximately 21 percent. This percentage has remained relatively stable throughout modern history due to a balance between the processes of photosynthesis, which produces oxygen, and respiration, which consumes it. Plants and certain microorganisms release oxygen as a byproduct of photosynthesis, while animals and other organisms consume oxygen for respiration. This equilibrium is crucial for maintaining the levels of oxygen that most life forms on Earth require.

Options indicating lower or higher oxygen percentages do not reflect the actual composition of Earth's atmosphere, which is primarily nitrogen (about 78 percent) and includes a steady proportion of oxygen. Thus, the choice indicating 21 percent accurately reflects the atmospheric composition critical for supporting life.