Final answer:
The subject of the question is viruses, which are categorized as c) Viruses. They are acellular, require a host to reproduce, and are considered obligate intracellular parasites.
Step-by-step explanation:
The correct answer to this question is c) Viruses. These entities are not living organisms, cannot grow, metabolize, or reproduce on their own, and have no cellular structure. To replicate and produce progeny, viruses must infect a host's cell and utilize its machinery. They are considered obligate intracellular parasites. An individual virus particle is known as a virion, and it requires the internal organelles and metabolic processes of the host to synthesize proteins and to make copies of itself. Despite being acellular and lacking metabolic processes, viruses possess genetic material and can evolve, which some argue gives them a form of 'life'.
Viruses are incredibly diverse, able to infect various organisms including humans, animals, plants, bacteria, and fungi. Though they are not included in the taxonomic tree of life, their impact on living organisms is significant, often resulting in diseases.