The simplest units of living matter that can maintain life and reproduce themselves. The human body contains 75 trillion of them.

• A. Tissue
• B. Organ System
• C. Organ
• D. Cell

Answer: (C)

Cells are the fundamental units that carry out life processes and can reproduce themselves. They’re the smallest units capable of sustaining life on their own, and organisms are made up of many cells that work together. The human body contains trillions of cells, each tailored to a function but all able to maintain life.

Tissues are formed when groups of similar cells come together to perform a common function; organs are larger structures made of different tissues that work together to perform specific tasks; organ systems are even bigger, consisting of multiple organs that coordinate broader physiological roles. Because the question asks for the simplest unit that can maintain life and reproduce, the description points to the cell. An organ, tissue, or organ system cannot by itself carry out all life-sustaining processes; they’re built from cells and depend on them to function.