Which of the following is defined as a collection of similar cells that perform a specific function?

The correct choice is tissue because it refers to a group of similar cells that work together to carry out a specific function within the body. Tissues are essential as they combine to form organs and contribute to the overall functioning of organ systems. For example, muscle tissue consists of muscle cells that contract to enable movement, while connective tissue provides support and binds other tissues together. Understanding the concept of tissue is vital in the study of anatomy and physiology, as it lays the groundwork for more complex structures within the body.

In contrast, an organ is made up of multiple types of tissues working together, while an organ system consists of various organs that coordinate to perform a broader range of functions. A single cell is the basic unit of life and does not perform specialized functions as a collective group would. Therefore, tissue is the most accurate choice for this question.