Which structural unit in an organism consists of a collection of tissues performing specialized functions?

The correct answer is the structural unit known as an organ. An organ is composed of two or more types of tissues that work together to perform specific tasks necessary for the organism's survival and functionality. For example, the heart is an organ made up of muscle tissue, connective tissue, and epithelial tissue, all collaborating to facilitate the pumping of blood throughout the body.

In contrast, a tissue consists of a group of similar cells that work together to perform a particular function, but it does not encompass the variety of tissues required to create an organ. A system refers to a group of organs that work together to perform complex functions, such as the circulatory or respiratory systems. An organism represents the entire living entity, made up of various systems, organs, and tissues. Understanding these definitions helps clarify why the organ is the correct answer as it uniquely fits the description of a structural unit defined by a collection of tissues with specialized functions.