**Core Concept**
Active immunity is a type of immunity that occurs when the body produces antibodies in response to direct exposure to an antigen, such as through vaccination or infection. This process involves the activation of the immune system, including the production of memory cells that can recognize and respond to the same antigen in the future.

**Why the Correct Answer is Right**
Injectable tetanus toxoid (TT) is a vaccine that contains a modified form of the tetanus toxin. When administered, the TT stimulates the body's immune system to produce antibodies against the tetanus toxin. This process is an example of active immunity because the body is actively producing antibodies in response to the antigen, rather than simply receiving pre-formed antibodies from an external source. The immune system also retains "memory" of the antigen, allowing for a rapid and effective response if exposed to tetanus toxin in the future.

**Why Each Wrong Option is Incorrect**
**Option B:** Passive immunity refers to the transfer of pre-formed antibodies from one individual to another, such as through breastfeeding or immunoglobulin injections. Injectable TT does not involve the transfer of pre-formed antibodies.

**Option C:** Native immunity is not a recognized term in immunology. The correct terms are innate immunity and adaptive immunity.

**Option D:** Reaction immunity is not a valid term in immunology. The correct terms are active immunity and passive immunity.

**Clinical Pearl / High-Yield Fact**
It's essential to note that active immunity can take several days to develop after vaccination, which is why booster shots are often given to maintain immunity levels.

**Correct Answer:**
✓ Correct Answer: A. Active immunity