How is an organ defined?

An organ is defined as a group of tissues that work together to perform specific functions within the body. This definition is grounded in the hierarchy of biological organization, where cells group to form tissues, and tissues then combine to create organs. Each organ has multiple types of tissues that collaborate to carry out complex activities essential for the organism's survival. For instance, the heart is an organ composed of muscle tissue, connective tissue, and nervous tissue, all functioning together to pump blood.

The other options describe different biological concepts. A group of rapidly dividing cells refers to a process often associated with cell growth or cancer rather than the structural and functional complexity of organs. A single type of tissue performing one function describes tissues, not organs, as organs are made of multiple tissue types. Lastly, a collection of bodily fluids pertains to substances like blood or interstitial fluid, which are not considered organs, as they do not consist of tissues working together. Hence, identifying an organ specifically as a collaboration of tissues is fundamental to understanding its role in the body.