What is the functional grouping of cells known as?

The correct answer is tissue because it refers to a group of cells that share a common structure and function. Tissues are composed of similar cells that work together to perform a specific activity, such as muscle tissue that facilitates movement or epithelial tissue that covers and protects surfaces. In the context of biology, understanding tissues is crucial as they serve as the building blocks that form larger structures like organs and organ systems.

Other options, like organ, refer to a structure made up of different types of tissues working together to perform specific functions, while an organ system consists of multiple organs that work in conjunction to carry out complex processes within an organism. The term structure is more generic and does not specifically denote a grouping of cells that perform a function. Thus, focusing on the definition of tissue clarifies its importance as a functional grouping within biological systems.