What is an organ comprised of?

An organ is defined as a group of tissues combined together to perform specific functions in the body. This means that various types of tissues, such as muscle tissue, connective tissue, and epithelial tissue, work in harmony to fulfill the organ's role in the body's functioning. For example, the heart is made of cardiac muscle tissue, connective tissues, and nerve tissues, all working together to pump blood throughout the body.

The other choices describe different biological structures but do not accurately convey what an organ is. A collection of discrete cells, while important at a more microscopic level, does not encompass the organization and function of tissues that form an organ. A mixture of different cells suggests a random assortment, which would not necessarily create a functional unit like an organ. Lastly, a series of functions performed by the body refers more broadly to physiological processes rather than the structural composition of an organ. Understanding that organs are built from specific combinations of tissue types gives clarity to their functional roles in the human body.