Final answer:
The plasmid ring is not part of a bacteriophage.
Step-by-step explanation:
In the context of a bacteriophage, the structure that is not part of a bacteriophage is B. Plasmid ring. A bacteriophage consists of a protein coat (capsid), genetic material (DNA or RNA), and an injection tube. The plasmid is a small ring of DNA found in bacteria, but it is not a part of the structure of a bacteriophage.