Which of the following is not a major class of biological units?

Cytosol is the aqueous component of the cytoplasm in a cell, where various organelles and other cell structures are suspended. While it plays a critical role in cellular functions, it is not a distinct class of biological units like the others listed.

Prokaryotic cells are characterized by the absence of a nucleus and membrane-bound organelles, while eukaryotic cells contain a nucleus and are more complex, with various organelles. Unicellular organisms consist of a single cell that can perform all necessary life functions independently, representing a category of living organisms. Each of these represents a distinct and recognized category of biological life forms, whereas cytosol refers to a part of the cellular structure rather than a classification of biological units. Thus, identifying cytosol as not being a major class aligns with the definitions of the terms involved.