Final answer:
A virus that infects bacteria is known as a bacteriophage. Bacteriophages can infect bacterial cells and replicate using either the lytic or lysogenic cycles, with examples including T4 and lambda viruses respectively.
Step-by-step explanation:
A virus that infects bacteria is called a bacteriophage. Bacteriophages are viruses that bind to surface receptors on a bacterial cell to inject their genome, initiating an infection. This can result in a productive infection, where new virus particles are created and released as the cell bursts in a lytic cycle, such as with the T4 bacteriophage that infects Escherichia coli.
Alternatively, the bacteriophage may enter a lysogenic cycle, where the viral genome integrates with the host's DNA, becoming a prophage, and replicates along with the host cell's genome without harming it. An example of this is the lambda virus. Thus, the correct answer to the question is B: bacteriophage.