Final answer:
A surface protein, often a glycoprotein, is the part of the virus responsible for recognizing and attaching to specific host cells, thus initiating infection.
Step-by-step explanation:
The viral structure responsible for recognizing specific host cells is A. a surface protein. Viruses have a basic structure which includes a protein coat called the capsid, and they use glycoproteins to attach to their host cells at molecules known as viral receptors. This attachment mechanism is essential for a virus to infect a cell and is dictated by the interaction of viral surface proteins with receptors on the host cell's surface. Glycoproteins can be found embedded in the envelopes of viruses such as HIV or as spikes on non-enveloped viruses, like adenoviruses, and allow viruses to specifically recognize and infect host cells.