The ultimate source of energy for plants comes from the sun.
Plants are autotrophs, meaning they produce their own food through a process called photosynthesis. During photosynthesis, plants use sunlight to convert carbon dioxide and water into glucose and oxygen. The sunlight acts as the driving force behind this process, providing the necessary energy for the conversion of inorganic materials into organic food. Without sunlight, plants would not be able to create the energy they need to grow, which in turn affects the entire food chain. Therefore, the correct answer is b) sun.