Which term describes a functional grouping of tissues?

The term that describes a functional grouping of tissues is "organ." An organ consists of different types of tissues that work together to perform specific functions within the body. For example, the heart is an organ made up of muscle tissue, connective tissue, and epithelial tissue, all coordinating to pump blood. Understanding this concept is crucial in grasping how complex biological systems are structured and how they function cohesively.

In contrast, the other options refer to different biological concepts. A compound generally relates to chemistry and the combination of elements, while a system refers to a higher level of organization involving multiple organs working together, such as the circulatory system. Cellular structure pertains to the organization and components of cells, which is a smaller scale than what is defined as an organ. Therefore, "organ" is the most accurate term for a functional grouping of tissues.