Different essential nutrients like nitrogen, carbon, oxygen and water are cycled and changed from one form to another in separate biogeochemical cycles in an ecosystem. The producers convert the solar energy received from the Sun into chemical energy and also fix up the nutrients from the soil, and make them available to the higher trophic levels in the ecosystem.

The correct answer is option 'A' - Energy flow.

Explanation:
An ecosystem is a community of living organisms (plants, animals, and microorganisms) interacting with each other and their physical environment. Ecosystems are characterized by the flow of energy, nutrients, and gases.

- Energy flow: Energy is the driving force that sustains life in an ecosystem. It flows through the ecosystem in the form of sunlight, which is converted into chemical energy by plants through photosynthesis. This energy is then transferred to herbivores when they consume plants, and further transferred to carnivores when they consume herbivores. Energy flow is essential for the functioning of an ecosystem, as it powers all biological processes and allows organisms to grow, reproduce, and carry out their daily activities.

- Nutrients flow: Nutrients are essential for the growth and survival of organisms in an ecosystem. They include elements such as carbon, nitrogen, phosphorus, and potassium, among others. Nutrients flow through the ecosystem in a cycle known as the nutrient cycle. Plants take up nutrients from the soil, and when they are consumed by animals, these nutrients are transferred to the animal's body. When organisms die and decompose, their nutrients are returned to the soil, where they can be taken up by plants again. This continuous flow of nutrients ensures the availability of essential elements for the different organisms in the ecosystem.

- Gaseous flow: Gases such as oxygen, carbon dioxide, and nitrogen are also important components of an ecosystem. Oxygen is necessary for the respiration of organisms, while carbon dioxide is essential for photosynthesis in plants. Nitrogen is a vital element for the formation of proteins and DNA. These gases flow through the ecosystem, being exchanged between organisms and the environment. For example, plants release oxygen during photosynthesis, which is then used by animals for respiration. Animals, in turn, release carbon dioxide, which can be used by plants for photosynthesis. Nitrogen is cycled through various processes such as nitrogen fixation, nitrification, assimilation, and denitrification. The gaseous flow ensures the availability of these essential gases for the organisms in the ecosystem.

In summary, the function of ecosystems includes the flow of energy, nutrients, and gases. These flows are interconnected and essential for the survival and functioning of the organisms within the ecosystem.