**Core Concept**
The Mumps vaccine is a live attenuated vaccine that contains a weakened strain of the mumps virus. This strain is crucial for inducing immunity against the virus without causing the full-blown disease.

**Why the Correct Answer is Right**
The correct answer is the specific strain of the mumps virus used in the vaccine. The strain is attenuated, meaning it has been weakened to the point where it cannot cause significant disease in healthy individuals. The vaccine contains the Jeryl Lynn strain, which was isolated from the daughter of a virologist. This strain has been extensively studied and is known for its ability to induce a strong immune response.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the Rubella virus is not used in the Mumps vaccine. The Rubella virus is used in the Measles, Mumps, and Rubella (MMR) vaccine, but not as the strain used in the Mumps vaccine.
**Option B:** This is incorrect because the Jeryl Lynn strain is the correct strain used in the Mumps vaccine. The other options are not recognized strains of the mumps virus.
**Option D:** This is incorrect because the Schwarz strain is actually used in the Rubella vaccine, not the Mumps vaccine.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Jeryl Lynn strain of the mumps virus is used in the Mumps vaccine, and it's a live attenuated vaccine that induces immunity without causing the full-blown disease.

**Correct Answer: C. Jeryl Lynn strain**