**Core Concept**
Antigenic drift refers to the gradual, continuous change in the surface proteins of viruses, particularly influenza viruses. This change occurs due to minor mutations in the viral genome, leading to a shift in the virus's antigenic profile. As a result, the immune system may not recognize the new viral strain, making it harder for the body to mount an effective immune response.

**Why the Correct Answer is Right**
Antigenic drift is a key mechanism of viral adaptation, allowing influenza viruses to evade the host's immune system. This process is facilitated by the high error rate of RNA polymerase, which can introduce mutations into the viral genome. As a result, the surface proteins of the virus, such as hemagglutinin (HA) and neuraminidase (NA), undergo subtle changes, making it more difficult for the immune system to recognize and neutralize the virus. The correct answer is **Influenza virus**, as it is the primary example of a virus that exhibits antigenic drift.

**Why Each Wrong Option is Incorrect**
**Option A:** HIV - While HIV does undergo genetic mutations, antigenic drift is not a primary mechanism of its adaptation.
**Option B:** Rotavirus - This virus primarily undergoes antigenic shift, a more drastic change in its genome.
**Option C:** Measles virus - Although measles virus does exhibit some antigenic variation, it is not primarily due to antigenic drift.

**Clinical Pearl / High-Yield Fact**
Antigenic drift is a key reason why annual influenza vaccines are updated to reflect the current circulating strains of the virus.

**Correct Answer: C. Measles virus**