Which of the following best defines an organ?

The definition of an organ encompasses a collection of different tissues that work together to carry out a specific function or set of functions within a biological system. In this context, an organ is more than just a single tissue or cell; it represents an organizational level in the body's complexity. For instance, the heart is made up of muscle tissue, connective tissue, and epithelial tissue, all of which collaborate to enable the heart to pump blood efficiently.

This understanding of an organ highlights its role in the larger system of the body, where organs like the heart or lungs interact within organ systems. These systems, which include the circulatory and respiratory systems, are essential for maintaining homeostasis and supporting life.

In contrast, the other definitions provided do not accurately capture the complexity and functional integration seen in organs. A single cell does not represent the collaborative nature of an organ, nor does a group of similar cells that form a tissue. Additionally, while atoms form the building blocks of matter, they do not fulfill the biological criteria that define living structures such as organs.