**Core Concept**
The first immunization of a baby, also known as the first dose of immunization, is a critical step in preventing infectious diseases and ensuring public health. This concept is based on the principles of immunology, where the introduction of antigens triggers an immune response, providing long-term protection against specific pathogens.

**Why the Correct Answer is Right**
The correct answer is related to the administration of BCG (Bacillus Calmette-Guérin) vaccine to newborns. BCG vaccine is a live attenuated vaccine that protects against tuberculosis (TB), a serious bacterial infection affecting the lungs and other parts of the body. The BCG vaccine is administered via intradermal injection, usually in the upper arm, to stimulate an immune response against TB. This vaccine is particularly crucial in regions with high TB prevalence.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not directly relate to the concept of first immunization of a baby. While vaccines like OPV (Oral Poliovirus Vaccine) are crucial for preventing polio, they are not typically administered as the first dose of immunization.
* **Option B:** This option is incorrect because Hepatitis B vaccine is an essential vaccine for preventing Hepatitis B infection, but it is not the first immunization administered to babies.
* **Option C:** This option is incorrect because DPT (Diphtheria, Pertussis, and Tetanus) vaccine is a combination vaccine that protects against three serious bacterial infections. However, it is not the first vaccine administered to newborns.

**Clinical Pearl / High-Yield Fact**
The BCG vaccine is administered at birth in many countries, particularly in regions with high TB prevalence, to provide early protection against TB. This highlights the importance of early immunization in preventing infectious diseases.

**Correct Answer:** C. BCG vaccine