Final answer:
The correct answer is A. Host. Viruses are obligate intracellular parasites requiring a host to reproduce, and this relationship exemplifies parasitism.
Step-by-step explanation:
The organism in/on which smaller organisms or viruses live, feed, and reproduce is known as the host. In the context of viruses, they are referred to as obligate intracellular parasites because they must infect a cellular host to survive and replicate. Viruses, while not classified as living organisms because they lack cellular structures, do possess genetic material and the ability to evolve. They invade the host cell and utilize its machinery to produce viral components, leading to the formation of new virus particles termed virions.
Parasitism is a relationship in which a parasite lives on or in a host organism and derives nutrients at the host's expense. This interaction typically harms the host by draining resources needed for its own survival, without necessarily causing immediate death, as this would not favor the parasite's reproductive cycle. It is worth noting that the term pathogen is used to describe an organism or infectious agent that causes disease, which can include parasites.