Final answer:
The structures that bind to receptor molecules on the host cell membrane in the context of HIV are glycoproteins, represented as letter A in the choices provided.
Step-by-step explanation:
In the context of HIV, a retrovirus, its ability to bind to host cell membranes is facilitated by structures known as glycoproteins. Within the options presented, letter A marks fungiform structures on the viral envelope, which correspond to these glycoproteins. Specifically, HIV uses glycoproteins gp120 and gp41 that are embedded in its envelope. These structures play a critical role by binding to CD4 receptors on the surface of immune cells, such as T lymphocytes. Doing so, HIV is able to attach to and subsequently fuse with the host cell membrane, allowing the virus to release its contents into the cell, including its single-stranded RNA genome. Viral enzymes then reverse transcribe this RNA into DNA, which can be incorporated into the host genome, leading to the production of new viral particles.
Answering the original question, the structures that bind to receptor molecules on the host cell membrane are HIV's glycoproteins, represented as letter A in the given choices. These virus-specific glycoproteins have evolved to exploit host cell receptors to facilitate virus entry and subsequent infection. It is a sophisticated mechanism that not only defines the infectious process of HIV but also highlights the importance of understanding viral structure for the development of antiviral therapies.