Final answer:
The smallest and most numerous structural units possessing the basic characteristics of life are called cells, which are the fundamental building blocks of all living organisms.
Step-by-step explanation:
The smallest and most numerous structural units possessing the basic characteristics of life are called cells. Cells are fundamental for all living organisms and are the building blocks from which tissues, organs, and systems are formed. In the context of biological organization, cells are followed by tissues, which are groups of similar cells that work together to perform a function; organs, which are made up of tissues to perform specific functions; and systems, which are collections of organs that work together to perform complex functions.