The correct answer is option 2: Energy. In an ecosystem, energy flows in a unidirectional manner. This means that energy enters the ecosystem from the sun in the form of sunlight, and it is then transferred from one organism to another as they interact with each other. This energy flow is essential for the survival and functioning of the ecosystem. In contrast, options 1, 3, and 4 (Light, Water, and Biomass) do not exhibit unidirectional flow in the same way as energy does. Light can be reflected or absorbed, water can be recycled through the water cycle, and biomass can be decomposed and reused. Therefore, option 2 correctly represents the unidirectional flow of energy in an ecosystem.