Which process do plants primarily use to convert sunlight into energy?

Plants primarily use photosynthesis to convert sunlight into energy, which is essential for their growth and development. This process takes place mainly in the chloroplasts of plant cells, where chlorophyll captures sunlight. In photosynthesis, carbon dioxide from the air and water from the soil are combined, using the energy from sunlight, to produce glucose (a form of sugar that serves as energy) and oxygen. This not only fuels the plant's activities but also contributes to the oxygen supply in the atmosphere, benefiting other living organisms.

Other processes mentioned in the options serve different purposes. Cellular respiration, for instance, is a process by which plants (and all living organisms) break down glucose to release energy, but it occurs after photosynthesis has produced glucose. Transpiration is related to the movement of water through a plant and the loss of water vapor from its aerial parts, crucial for nutrient transport and temperature regulation but not for energy conversion. Respiration is a broader term that encompasses both cellular respiration and the general metabolic processes that occur in cells, primarily focusing on energy release rather than energy capture. Thus, photosynthesis stands out as the fundamental process by which plants convert sunlight into usable energy.