**Core Concept**
The question is testing the understanding of the natural barriers that prevent the transmission of rabies virus. The rabies virus is primarily spread through the saliva of infected animals, usually through bites. The natural barriers that prevent transmission include the skin and mucous membranes, which act as a physical barrier, and the presence of antibodies in the saliva that neutralize the virus.

**Why the Correct Answer is Right**
The most effective natural barrier to rabies is the skin, particularly the intact skin barrier. If an animal bites a person and the skin is broken, the rabies virus can enter the body and cause infection. However, if the skin is intact, the virus is unable to penetrate and cause infection. This is because the skin is a physical barrier that prevents the virus from coming into contact with the nerve endings, which are the target of the virus.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while mucous membranes do provide some protection against rabies, they are not the most effective natural barrier.
* **Option B:** This option is incorrect because saliva itself is not a barrier to rabies, but rather a means of transmission.
* **Option C:** This option is incorrect because while antibodies in the saliva can neutralize the virus, they are not a physical barrier to prevent transmission.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the incubation period of rabies can range from a few days to several years, but the disease is almost always fatal if left untreated. This highlights the importance of prompt medical attention and post-exposure prophylaxis (PEP) in cases of animal bites.

**Correct Answer:** **D**. Skin