Final answer:
Cells are the least specialized yet most numerous level of biological organization, serving as the building blocks for more complex structures like tissues, organs, and organ systems.
Step-by-step explanation:
The level of organization that is the least specialized yet most numerous is cells. Cells are the basic building blocks of all organisms, including the most complex ones. Atoms combine to form molecules, which in turn make up cell components or organelles. In multicellular organisms, similar cells form tissues. Tissues collaborate to create organs, and organs work together to form organ systems. Although cells may become specialized in multicellular organisms, they remain the most fundamental and abundant unit of biological organization.