What type of viruses require host cell machinery to replicate?

Obligate intracellular viruses are those that must enter a host cell in order to replicate. These viruses are entirely dependent on the host's cellular machinery and metabolic processes for their replication and propagation. They lack the necessary components and capabilities to reproduce independently outside of a host organism. This reliance is a defining characteristic of these viruses; they typically inject their genetic material into the host cell, hijacking the host's ribosomes, enzymes, and other biochemical pathways to synthesize new viral components and assemble new virions.

In contrast, other types of viruses presented in the choices do not encapsulate the essence of needing a host for their replication. Free-living viruses are not an accepted concept in virology, as all viruses are parasitic by nature. Autonomous viruses suggest a degree of self-sufficiency that is inaccurate since all viruses ultimately require a host for replication. Transmissible viruses may refer to viruses that can be passed from one host to another, but this does not encapsulate the requirement of host machinery for viral replication. Thus, the concept of obligate intracellular viruses is pivotal in understanding viral life cycles and their dependence on host cells.