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 within an organism that work together to execute a specific function. This is a fundamental concept in biology as it establishes a clear delineation of how cells cooperate to form more complex structures. Tissues are essential for the functioning of organs, as they provide the necessary support, structure, and specialized functions that contribute to the overall operation of the organ.

In the context of the other choices, it's important to note that while an organ does consist of tissues and serves a specific purpose, the description provided in that choice is imprecise, as it implies organs work collectively rather than being part of a single structure. The third choice accurately describes a cell but focuses more on replication rather than its role in tissue and organ systems. Lastly, the fourth option describes an organism at a higher level of organization but doesn't capture the specifics of how the levels (cells, tissues, organs) are defined. Therefore, the first choice most accurately encapsulates the essence of what a tissue is in the hierarchy of biological organization.