What are structures composed of specialized tissues and performing specific functions?

The correct answer is organs because they are composed of specialized tissues that work together to perform specific functions within the body. Each organ, such as the heart, lungs, or skin, is made up of multiple types of tissues which collaborate to carry out distinct roles essential for the organism's overall health and functioning. For instance, the heart consists of muscle tissue that contracts to pump blood, connective tissue that provides structure, and nerve tissue that helps regulate heartbeats.

Cells are the basic building blocks of all living organisms, but they do not perform the complex functions associated with organs on their own. Tissues are groups of similar cells that work together to perform a specific function, but they are one step below organs in organizational complexity. Systems are collections of interconnected organs that work together to perform broader functions within the body, which means they encapsulate a larger scale than individual organs.