What is defined as a structure composed of specialized tissue that performs specific functions?

The correct choice is "Organ," which refers to a structure made up of specialized tissues that work together to perform specific functions within an organism. Organs are vital components of biological systems, and each organ has a unique role that contributes to the overall health and operation of the body. For example, the heart is an organ designed to pump blood, while the lungs are responsible for gas exchange.

In contrast, tissue refers to a group of similar cells that work together to perform a specific function but does not encapsulate the defined structural complexity of an organ. A cell is the basic unit of life, smaller than an organ, and carries out its life processes independently, but it cannot perform complex functions on its own like an organ can. Moreover, a system is a group of organs that work together for a common purpose, indicating a higher level of organization beyond a single organ. Therefore, the definition of an organ aligns accurately with the description provided in the question.