Final answer:
The cellular level is the simplest in complexity among the cellular, organismal, and populational levels, being the basic unit of life.
Step-by-step explanation:
The simplest level of complexity among the cellular, organismal, and populational levels is the cellular level. This level consists of the basic unit of life, the cell. Cells are the most fundamental structural and functional units in living organisms, as all life functions of organisms occur within cells. Multicellular organisms have cells that combine to form tissues, and these tissues come together to create organs and organ systems, adding to the complexity of the organism. Populations are even more complex, as they encompass all individuals of a species living within a specific area and interacting with their environment.