Final answer:
The answer is d) Receptor. Viral attachment is mediated by specific interactions between viral glycoproteins and receptors on the host cell's membrane.
Step-by-step explanation:
The correct answer to the question "The viral ______ plays a role in attaching a virion to the host cell." is d) Receptor. Viruses attach to a specific receptor site on the host cell membrane through proteins in the capsid or via glycoproteins embedded in their viral envelope.
The specificity of these interactions allows a virus to bind to particular host cells. For example, the HIV virus uses glycoproteins embedded in its envelope to attach to host cells. This interaction is akin to a key fitting into a specific lock, indicating that viruses can only infect cells with the appropriate receptor that matches their attachment proteins.