**Core Concept**
Antigenic shift is a process in which two different influenza viruses infect the same cell, leading to the creation of a new virus with a combination of genes from both parent viruses. This results in a significant change in the virus's antigenic properties, making it a novel strain to which the human population may have little or no immunity.

**Why the Correct Answer is Right**
The process of antigenic shift occurs due to the reassortment of genes between two different influenza A viruses. This can happen when a person is infected with two different influenza A viruses, either simultaneously or sequentially, allowing the viruses to exchange genetic material and create a new virus. The new virus can have a combination of surface proteins (hemagglutinin and neuraminidase) from both parent viruses, leading to a significant change in its antigenic properties.

**Why Each Wrong Option is Incorrect**

* **Option A:** This option is not relevant to antigenic shift, as it does not describe the process of gene reassortment between two different influenza viruses.
* **Option B:** This option is incorrect because antigenic drift, not antigenic shift, refers to the gradual change in the virus's surface proteins due to point mutations.
* **Option C:** This option is incorrect because it describes the process of antigenic drift, not antigenic shift.

**Clinical Pearl / High-Yield Fact**
Antigenic shift is a significant mechanism of influenza virus evolution, leading to the emergence of new pandemic strains to which the human population may have little or no immunity. This highlights the importance of annual influenza vaccination to protect against the most common circulating strains.

**Correct Answer:** C.