Which choice refers to the basic structural and functional units of all living organisms?

The basic structural and functional units of all living organisms are cells. Cells are often referred to as the "building blocks of life" because they perform all the necessary functions that sustain life. Each cell contains the essential components for carrying out activities such as metabolism, reproduction, and response to stimuli, making them fundamental to all biological processes.

In multicellular organisms, cells work together to form tissues, which are collections of similar cells that perform specific functions. These tissues then combine to form organs, which are structures made up of different types of tissues that work together for complex functions. Systems are even more complex, consisting of groups of organs that collaborate to perform overall functions necessary for the survival of the organism. Understanding that cells are the foundational units helps clarify the hierarchy of biological organization—from cells to tissues to organs to systems.