Photosynthesis is a process in which plants, algae, and some bacteria convert light energy into chemical energy to synthesize food molecules. The correct option is option 4, sunlight. Sunlight is essential for photosynthesis as it provides the energy needed to drive the reaction. Chlorophyll, a pigment found in the chloroplasts of plants and algae, absorbs light energy. This energy is then used to convert water (option 1) and carbon dioxide (option 3) into glucose and oxygen. While water and carbon dioxide are necessary for photosynthesis, they cannot initiate the process without the presence of sunlight. Nutrients (option 2) are required for plant growth and development but are not directly involved in photosynthesis. Therefore, option 4, sunlight, is the correct answer as it is an essential component for photosynthesis.