**Core Concept**
Active immunity is a type of immunity where the body produces antibodies in response to direct exposure to an antigen, either through infection or immunization. This results in long-term protection against future infections.

**Why the Correct Answer is Right**
Active immunity is achieved through the activation of immune cells, such as B cells and T cells, which recognize and respond to specific antigens. The B cells produce antibodies that bind to the antigens, marking them for destruction and neutralizing the pathogen. This process involves the activation of various immune cells and the production of cytokines, which coordinate the immune response. The resulting antibodies provide long-term protection against future infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because passive immunity, not active immunity, is acquired through the transfer of pre-formed antibodies from one individual to another, such as through mother's milk or immunoglobulin injections.
* **Option B:** This option is incorrect because tolerance, not active immunity, is a state of unresponsiveness to an antigen, often due to the deletion of autoreactive T cells or the generation of regulatory T cells.
* **Option C:** This option is incorrect because immunological memory, while related to active immunity, is a specific aspect of the immune response that allows for rapid and efficient response to future infections.

**Clinical Pearl / High-Yield Fact**
Active immunity can be achieved through various means, including vaccination, which stimulates the body to produce antibodies and immune cells that recognize and respond to specific pathogens.

**Correct Answer: A. Active immunity is achieved through the activation of immune cells, such as B cells and T cells, which recognize and respond to specific antigens.**