**Core Concept**
The question is testing knowledge of a specific salivary protein that has been shown to have antiviral properties, specifically against HIV. This protein is an example of a salivary enzyme that plays a role in protecting against the transmission of infectious diseases.

**Why the Correct Answer is Right**
The correct answer is secretory IgA, a type of antibody that is found in saliva and other mucosal secretions. Secretory IgA works by binding to the HIV virus and neutralizing its ability to infect cells. This is achieved through the binding of the Fc region of IgA to the HIV envelope protein, which prevents the virus from entering host cells. The presence of secretory IgA in saliva is thought to be an important factor in protecting against the transmission of HIV through saliva.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because there is no evidence to suggest that lysozyme has any significant role in preventing the transmission of HIV.
* **Option B:** This option is incorrect because lactoferrin is a protein that has antimicrobial properties, but it is not specifically known for its ability to prevent the transmission of HIV.
* **Option C:** This option is incorrect because amylase is an enzyme that breaks down starches into sugars, and it has no known role in preventing the transmission of HIV.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that the presence of secretory IgA in saliva is an important factor in protecting against the transmission of infectious diseases, including HIV. This is why saliva is often used as a diagnostic fluid in tests for infectious diseases.

**Correct Answer: D. Secretory IgA**