**Core Concept**
The MMR (Measles, Mumps, and Rubella) vaccine is a live, attenuated vaccine that provides immunity against three viral infections. Live, attenuated vaccines contain a weakened or mild form of the virus, which stimulates a strong immune response without causing the full-blown disease.

**Why the Correct Answer is Right**
The MMR vaccine contains a weakened form of the measles, mumps, and rubella viruses. When administered, these attenuated viruses replicate in the body, inducing an immune response that includes the production of antibodies and the activation of immune cells. This immune response provides long-term protection against future infections with the same viruses.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the MMR vaccine is not a killed-virus vaccine. Killed-virus vaccines contain a completely inactivated form of the virus, which does not replicate in the body.
* **Option B:** This option is incorrect because the MMR vaccine is not a subunit vaccine. Subunit vaccines contain only specific components of the virus, such as proteins or sugars, which are used to stimulate an immune response.
* **Option C:** This option is incorrect because the MMR vaccine is not a conjugate vaccine. Conjugate vaccines combine a weakened form of the virus with a carrier protein to enhance the immune response.

**Clinical Pearl / High-Yield Fact**
The MMR vaccine is often administered in combination with other vaccines, such as the varicella vaccine (MMRV), to provide protection against multiple viral infections at once.

**Correct Answer: C. Live, attenuated vaccine.**