Which option is not a direct definition provided for organ system or organism?

• A. A group of organs working together to perform a function
• B. A living thing
• C. A single organ
• D. A living cell

Answer: (D)

Understanding levels of biological organization helps explain this. An organ system is a group of organs working together to perform a function, and an organism is a living thing. Describing a single organ doesn’t define an organ system or an organism—it’s just one component, not the whole system or the whole living being. Describing a living cell isn’t a definition of either organ system or organism either, because it points to the cell level, the basic unit of life, which can be part of a larger organism but does not by itself define the entire system or the whole organism. So the statement about a living cell is the one that does not directly define an organ system or an organism.