The main source of energy in an ecosystem is

Concept: In virtually every ecosystem, the ultimate energy input is solar radiation. Producers (green plants, algae, and cyanobacteria) capture sunlight through photosynthesis and convert it into chemical energy stored in organic molecules; that stored energy then flows in one direction through the food chain — from producers to primary consumers, to secondary consumers, and finally to decomposers — at each step some of it being lost as heat.

Application: For this question, among the four options only sunlight is an external energy input to the ecosystem: it is fixed by producers during photosynthesis and becomes the chemical energy that powers every subsequent trophic level.

Cross-check — the other three options are not energy sources at all:

• ATP is the energy-carrier molecule cells generate internally (largely via cellular respiration) to power their own biochemical reactions; it is produced from energy already captured, not the ecosystem's original energy input.

• DNA is the molecule that stores an organism's hereditary instructions; it encodes information for building and running cells, not energy for the ecosystem.

• RNA carries out protein synthesis by reading and translating those hereditary instructions; like DNA, it is an information-carrying molecule rather than an energy source.

Result: the main source of energy in an ecosystem is sunlight.