**Core Concept**
The hepatitis B vaccine induces a specific immune response that leads to the production of antibodies against the hepatitis B surface antigen (HBsAg). This vaccine is a recombinant DNA vaccine that uses a gene from the hepatitis B virus to produce a non-infectious form of the HBsAg, which is then used as the immunogen.

**Why the Correct Answer is Right**
The correct answer is related to the immune response generated by the hepatitis B vaccine. After immunization, the body produces antibodies against the HBsAg, which are detectable in the serum. These antibodies are primarily of the IgG class and are responsible for providing immunity against hepatitis B infection. The presence of these antibodies indicates a successful immune response to the vaccine and provides protection against future infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the hepatitis B vaccine or the expected immune response.
**Option B:** This option is incorrect because the presence of HBsAg in the serum would indicate an active infection, not a successful vaccination.
**Option C:** This option is not directly related to the hepatitis B vaccine or the expected immune response.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the presence of antibodies against HBsAg is not a guarantee of immunity, and individuals may still be susceptible to other forms of hepatitis B, such as acute infection or chronic infection from other sources.

**Correct Answer: C. Antibodies against HBsAg.**