Viruses are
A. Obligate parasites
B. Obligate saprophytes
C. Partial parasites
D. Facultative parasites

Hint: Association of organisms that benefit one of the partners at the expense of others is called parasitism. Viruses are non-cellular, microscopic infectious vehicles that can only replicate inside a host cell. From a biological perspective, viruses cannot be classified as either a living organism or non-living. This is due to the fact that they own certain defining features of living organisms and nonliving entities.

Complete answer:
> Obligate parasites – These parasites are totally dependent on the host for a specific stage of their life-cycle or the entire length of their lives. All viruses are obligate parasites because they lack metabolic mechanisms of their own to make energy or to synthesize proteins. That’s why they depend on host cells to carry out these important functions. So this is the correct option.
> Obligate saprophytes – viruses are not saprophytes. So it is the wrong answer.
> Partial parasites – viruses are fully dependent on the host, so it is the wrong option.
> Facultative parasites – it is also a wrong option.

The correct answer is option (A) that is obligate parasites.

Note:
The virus cannot replicate itself outside the host cell. This is because viruses do not possess the required cellular apparatus. Hence, they enter and connect themselves to the specific host cells then insert their genetic materials, and reproduce by using the host genetic material. And finally, the host cell cleaves open, releasing the new viruses. Lytic viruses kill their host cells but lysogenic viruses do not harm their host.