**Core Concept**
Antigenic variation is a process by which pathogens, such as viruses and bacteria, change their surface antigens to evade the host's immune system. This allows them to persist and cause recurrent infections.

**Why the Correct Answer is Right**
Antigenic variation is a crucial survival strategy for pathogens like influenza viruses, where the hemagglutinin (HA) and neuraminidase (NA) proteins on the surface of the virus undergo mutations, resulting in new antigenic variants. This process is driven by the error-prone nature of viral replication and recombination events. The immune system's inability to recognize and target these new variants leads to recurrent infections.

**Why Each Wrong Option is Incorrect**
**Option A:** While some bacteria, such as Neisseria meningitidis, exhibit antigenic variation, it is not exclusive to this bacterium.
**Option B:** Antigenic variation is indeed a characteristic of some RNA viruses, including influenza.
**Option C:** This option is correct because HIV is a retrovirus that exhibits a different type of immune evasion mechanism, primarily through its ability to mutate and develop resistance to antiretroviral therapy, rather than antigenic variation.

**Clinical Pearl / High-Yield Fact**
Antigenic variation is a key factor contributing to the complexity of vaccine development, as it necessitates frequent updates to match the circulating strains of a pathogen.

**Correct Answer: C. HIV**