Which of the following levels of organization is considered the simplest living unit?

Tissues are composed of groups of similar cells that work together to perform a specific function. While they are essential for the organization of biological systems, they are not the simplest unit, as they rely on the existence of cells to form. Thus, tissues represent a higher level of organization than cells.

Organs are structures made up of different types of tissues that collaborate to perform complex functions necessary for an organism's survival. Like tissues, organs are more complex than cells, as they consist of multiple cellular groups working in concert, and therefore cannot be considered the simplest living unit.

Organisms are entire living entities that can function independently, composed of multiple organ systems working together. This level of organization encompasses various cells, tissues, and organs, making organisms the most complex level of biological hierarchy and far beyond the simplest living unit.