In the process of transduction, what injects the DNA into a host cell?

Transduction is a method by which genetic material is transferred from one bacterium to another via a virus, specifically a bacteriophage. In this process, the bacteriophage infects a bacterial cell, where it can inject its DNA. Once inside, the viral DNA can either integrate into the host's genome or remain separate, ultimately leading to the production of new viral particles that may carry some of the host's DNA to a different bacterial cell during subsequent infections.

The reason the other options do not apply is that bacteria, plasmids, and fungi do not have the function of injecting DNA into host cells in the same manner as a virus does. Bacteria can transfer genetic material through other methods such as conjugation and transformation, plasmids are small circular DNA that can replicate independently within a bacterium but do not inject DNA, and fungi are eukaryotic organisms that interact with bacteria differently and are not involved in the transduction process. Therefore, the virus is the correct answer since it specifically performs the action of injecting DNA into a host cell during transduction.