The correct answer is option 1: Sun. In an ecosystem, the ultimate source of energy is the Sun. Sunlight provides energy for photosynthesis, which occurs in green plants. During photosynthesis, plants convert sunlight into chemical energy in the form of sugar, which is option 3. However, it is important to note that while sugar is produced in photosynthesis, it is not the source of energy but a product of the conversion process. ATP (adenosine triphosphate), which is option 4, is a molecule that stores and provides energy in cells. While ATP is an important energy carrier in organisms, it is not the source of energy in the ecosystem. The Sun is the primary source of energy that drives the entire food web and sustains life in the ecosystem.