Which of the following best describes carbon dioxide?

Carbon dioxide is best described as colorless, odorless, and naturally occurring in all water. This is because carbon dioxide is a gas that exists in the atmosphere and can dissolve in water, forming carbonic acid. In natural water bodies, it plays a crucial role in various biological and chemical processes, such as photosynthesis and respiration.

The presence of carbon dioxide in water is essential for aquatic life, as it is used by aquatic plants to produce oxygen during photosynthesis. Furthermore, it is a normal constituent of water and does not cause any adverse effects in typical concentrations found in nature.

In contrast to the other descriptions, carbon dioxide is not colored or scented, and while it can be associated with pollution in certain contexts, it is not exclusively found in polluted water. Additionally, it is not inherently harmful to aquatic life; rather, it can become detrimental at very high concentrations, which is not reflective of its general state in aquatic environments.