The correct answer is option 4, green plants. Green plants are the primary producers in most ecosystems and are responsible for the maximum fixation of solar energy through the process of photosynthesis. During photosynthesis, green plants convert sunlight, carbon dioxide, and water into glucose (a form of sugar) and release oxygen as a byproduct. This process allows green plants to store solar energy in chemical bonds of glucose, which is then used to fuel their own growth and provide energy to other organisms that consume them. While bacteria, protozoa, and fungi play important roles in ecosystems, they do not fix as much solar energy as green plants. Green plants possess specialized structures called chloroplasts that contain the pigment chlorophyll, enabling them to capture sunlight and convert it into chemical energy.