Final answer:
The correct answer is d. plants. Plants produce oxygen through photosynthesis.
Step-by-step explanation:
The correct answer is d. plants.
Plants produce oxygen through a process called photosynthesis, where they use sunlight, water, and carbon dioxide to create glucose and oxygen. During photosynthesis, plants release oxygen into the air as a byproduct.
According to studies, Earth's atmosphere lacked abundant free oxygen until about 2 billion years ago despite the presence of plants releasing oxygen by photosynthesis. Chemical reactions with Earth's crust removed the oxygen gas as quickly as it formed. However, as the plant population grew and geological activity increased, more oxygen was produced.