Final answer:
The focus of ecological study at the ecosystem level is on energy flow and nutrient cycles, including how energy is transferred across trophic levels and how nutrients are cycled through the biophysical system. Therefore correct option is A
Step-by-step explanation:
The focus of ecological study at the ecosystem level is primarily on energy flow and nutrient cycles. This involves understanding how organisms in an ecosystem acquire energy in various ways and how that energy is transferred across different trophic levels from producers to apex predators, with energy being lost at each stage. Important components in this study are the modeling of ecosystems through conceptual models, analytical models for linear food chains, and simulation models for more complex food webs.
Additionally, the cycling of mineral nutrients such as water, carbon, nitrogen, phosphorus, and sulfur are crucial for ecosystem structure and functioning, and these biogeochemical cycles are significantly impacted by human activities. Hence, the study of these cycles is vital for understanding and achieving ecological sustainability in human ecosystems.