Concept:Both Spirogyra and Volvox exist at the cellular level of organization.Explanation:Spirogyra is a filamentous alga where each cell performs all life functions independently.Volvox is a colonial alga made of individual cells that live together but do not form tissues.In both organisms, the cell itself is the basic unit of organization.They do not possess tissues, organs, or organ systems.Answer:C. cell