The correct answer is option 2: Sunlight. Sunlight provides the energy needed for the process of photosynthesis to occur in plants. During photosynthesis, plants use sunlight to convert carbon dioxide and water into glucose (a type of sugar) and oxygen. This process occurs in the chloroplasts of plant cells, specifically in the chlorophyll molecules. Chlorophyll, option 3, is the green pigment found in the chloroplasts that captures sunlight and helps initiate the process of photosynthesis. However, while chlorophyll is necessary for photosynthesis to take place, it does not directly provide the energy for the process. Water, option 1, is also essential for photosynthesis as it provides the hydrogen atoms necessary for the creation of glucose. Carbon dioxide, option 4, is a molecule used by plants as one of the building blocks for glucose, but it does not provide energy for photosynthesis.