The correct answer is option 1, biological system. An ecological system refers to a system in which living organisms, such as plants, animals, and microorganisms, interact with each other and with their physical environment. It involves the study of how organisms interact with their environment and with other organisms in order to survive and reproduce. This includes the study of the flow of energy and nutrients through different trophic levels, the adaptation and evolution of species, and the overall functioning of ecosystems. The term "biological system" accurately describes an ecological system as it emphasizes the importance of living organisms and their interactions within the system. Therefore, option 1 is the correct choice.