what is the source of energy in most ecosystems?\no animals\no plants\no oxygen\no sunlight

Brief Explanations:

In most ecosystems, sunlight is the primary energy - source. Plants use sunlight for photosynthesis to produce energy - rich compounds, which are then passed through the food chain. Animals obtain energy by consuming plants or other animals. Oxygen is a by - product of photosynthesis and is used in respiration, but it is not an energy source.

Answer:

D. sunlight