A group of tissues performing a particular function in the body forms a/anACellBTissueCOrganDOrgan system

Cell is the basic unit of life. A group of cells having same structure and performing same function forms tissue. A group of tissues performing a particular function in the body forms an organ. A group of organs that work together to maintain a stable internal environment and perform a particular function is called as organ system.

Thus, the correct answer is 'Organ.'