Final answer:
Ultracentrifugation can separate cellular components based on their mass, but the plasma membrane cannot be pelleted by this technique due to its lipid bilayer structure. Correct option is C.
Step-by-step explanation:
Ultracentrifugation is a technique used to separate cellular components based on their mass. In this technique, the cell fractions are subjected to progressively higher centrifugal force to separate particulate cell components. While the cytosol, microsomes, and enzyme complexes such as ribosomes can be pelleted by ultracentrifugation, the plasma membrane cannot be pelleted by this process because it is a lipid bilayer structure that does not have sufficient mass to be sedimented.