The most explicit description of this means of energy processing would be:
c. Photosynthesis.
Photosynthesis is the process by which plants, algae, and some bacteria use energy from light to convert carbon dioxide and water into organic compounds, such as sugars, and release oxygen as a byproduct. This protist species is described as consuming carbon dioxide and emitting oxygen, which is a hallmark of photosynthesis and therefore provides the most accurate description of its energy processing.