Which option defines an organ?

An organ is defined as a unit of related tissues working together to perform specific functions essential for the organism's survival and health. Different types of tissues come together to form an organ, which allows for more complex functions than any single tissue could achieve on its own. For example, in the heart, muscle tissue enables contraction, connective tissue supports and protects, and nervous tissue helps regulate the heartbeat, all working collaboratively to ensure the organ performs its vital role in pumping blood throughout the body.

The other options do not accurately reflect what constitutes an organ. A collection of various systems comprises multiple organs working together for a common purpose, rather than defining a single organ. A group of different cells without a function does not represent the organized structure and specific roles required for an organ. Lastly, a single tissue serving many functions is a characteristic more aligned with tissues themselves rather than the complex interactions necessary to qualify as an organ. Thus, the definition of an organ as a unit of related tissues accurately captures its role and function within the biological hierarchy.