The correct answer is: B. Sunlight
Sunlight is the ultimate source of energy for all ecosystems on Earth. It is used by plants to create food through photosynthesis. This food is then used by animals and other organisms to get the energy they need to survive.
ATP (adenosine triphosphate) is a molecule that stores energy. It is used by cells to power their activities. However, ATP is not the ultimate source of energy. It is created using energy from sunlight or other sources.
DNA (deoxyribonucleic acid) is a molecule that contains the genetic code for all living things. It is not a source of energy.
RNA (ribonucleic acid) is a molecule that is involved in many important biological processes, including protein synthesis. It is not a source of energy.
In conclusion, the source of energy in an ecosystem is sunlight.