The best evidence to support the student's claim that “Life is possible because of the unique mixture of gases that cycle through the Earth’s spheres” is a diagram illustrating how plants use carbon dioxide, water, and sunlight to produce food and oxygen gas.
This evidence directly relates to the cycling of gases, particularly carbon dioxide and oxygen, which are essential for life. It demonstrates the process of photosynthesis, which is crucial for the production of oxygen, a vital component for most life forms on Earth. Thus, this diagram highlights the importance of the gases that cycle through the Earth's systems and their role in sustaining life.