Which of the following correctly describes one of the levels of organization in an organism?

A tissue is indeed defined as a group of similar cells in an organism that work together to perform a specific function. This level of organization is fundamental in biology, as it represents a grouping that allows for greater specialization and efficiency in tasks, such as the respiratory or connective tissues. Each tissue type has distinct characteristics and roles within the organism, illustrating the complexity and hierarchical nature of biological systems.

The other options contain inaccuracies in their definitions. For example, an organ system involves multiple organs working together, thus different from the tissue level. Additionally, a cell does not encompass tissues but rather is a smaller unit that forms tissues. Lastly, an organ is made up of multiple tissue types rather than just a group of cells. Understanding these distinctions is essential for grasping the structure and function of living organisms.