Final answer:
Hemagglutinin is the influenza protein that governs host specificity by enabling the virus to bind to host cell receptors.
Step-by-step explanation:
The protein responsible for determining host specificity in influenza viruses is hemagglutinin. This surface glycoprotein enables the influenza virus to bind to the sialic acid receptors on respiratory epithelial cells, which facilitates the viral entry into the host cell through endocytosis. The influenza virus is enveloped and among its outer proteins, hemagglutinin plays a pivotal role in attachment to host cells, while neuraminidase is crucial for the release of progeny viruses from the host cell by cleaving sialic acid receptors.
Considering the other proteins listed, nucleoprotein is involved in the encapsulation of the viral RNA, the matrix protein plays a structural role in the virion assembly, and neuraminidase aids in the release of newly formed viruses. However, it is the hemaglutinin protein that is most critical for host specificity as it governs the initial interaction with host cells.
Therefore, the final answer to the question is: 2) hemagglutinin. This protein is the most responsible for host specificity because it allows the influenza virus to bind to and infect specific host cells.