**Core Concept:** Immunization is the process of introducing immunological stimuli to an individual to develop adaptive immune responses, thereby providing immunity against specific pathogens. In newborns, the initial exposure to antigens is called the "first immunization."

**Why the Correct Answer is Right:**
The "first immunization" refers to the initial exposure of an infant to antigens from vaccines, which triggers the development of adaptive immunity against those specific pathogens. This process strengthens the baby's immune system and protects them from life-threatening diseases.

**Why Each Wrong Option is Incorrect:**
A. **Option A:** This is incorrect because it pertains to the primary immune response, not the initial exposure to antigens.
B. **Option B:** This also pertains to the primary immune response, which is different from the initial exposure to antigens.
C. **Option C:** This is incorrect because it refers to the passive immunity provided by breast milk, not the initial exposure to antigens.
D. **Option D:** This pertains to the use of antibiotics or other medications, which is unrelated to the first immunization.

**Clinical Pearl / High-Yield Fact:**
The "first immunization" is essential for infants, as it provides them with the necessary immunity against preventable diseases like diphtheria, pertussis, tetanus, and hepatitis B. These immunizations are typically administered through a series of injections during the first year of life and are crucial for the child's overall health and development.

**Correct Answer:** C. Hepatitis B vaccine

The correct answer is option C, Hepatitis B vaccine. Hepatitis B vaccine is the appropriate term for the first immunization of an infant. It is part of the routine immunization schedule for newborns to provide immunity against Hepatitis B virus infection, a serious health concern that can lead to chronic liver disease or liver cancer. Administering the vaccine during infancy ensures that the infant's immune system is prepared to recognize and combat the Hepatitis B virus should they encounter it later in life. This practice helps in reducing the overall burden of Hepatitis B infection in the population.