## Core Concept
Postexposure immunization, also known as post-exposure prophylaxis (PEP), is a medical intervention provided after potential exposure to a pathogen to prevent the development of disease. This concept is crucial in the prevention of certain infectious diseases, especially those caused by viruses or bacteria with significant morbidity and mortality.

## Why the Correct Answer is Right
Postexposure immunization is specifically recommended for diseases where immediate immunity can be conferred to prevent the disease from developing. Among the options provided, **Rabies** is a classic example where postexposure prophylaxis (PEP) is administered. PEP for rabies includes immediate washing of the wound, administration of rabies immunoglobulin, and a series of vaccinations. This approach is highly effective in preventing the onset of rabies if administered promptly after a bite or exposure.

## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific details of the diseases listed, we can infer based on common practices. Diseases like **Hepatitis B** do offer postexposure prophylaxis, which includes hepatitis B immune globulin (HBIG) and the first dose of hepatitis B vaccine, but the question seems to point towards a condition where immediate action is typically required.
- **Option B:** Similarly, **Hepatitis A** and other diseases might have postexposure measures, but they are not as universally recognized for immediate postexposure immunization as rabies.
- **Option C:** This option cannot be evaluated without specific details but considering standard practices, immediate postexposure immunization is critical in diseases with high mortality if not promptly addressed.
- **Option D:** Again, specifics are needed for a thorough evaluation, but typically, diseases with available vaccines or immunoglobulins for postexposure prophylaxis are considered.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **Rabies postexposure prophylaxis (PEP)** is most effective when administered immediately after exposure. Delaying PEP can lead to the development of rabies, which is almost invariably fatal. Therefore, understanding the need for and the protocol of PEP, especially for rabies, is crucial for medical practitioners.

## Correct Answer Line
**Correct Answer: D. Rabies.**