**Core Concept**
Chimerism refers to the presence of two or more different cell lines derived from different zygotes in the same individual. This occurs when two fertilized eggs fuse together, or when cells from one fetus are transplanted into a twin.

**Why the Correct Answer is Right**
Chimerism is a rare phenomenon where two genetically distinct cell populations coexist within an individual. This can happen in the case of monozygotic twins (identical twins) where they develop from a single fertilized egg that splits in two, or in the case of dizygotic twins (fraternal twins) where cells from one fetus are transferred to the other, creating a mixture of cells. The presence of two cell lines can lead to different tissue types, such as skin, blood, or organ tissues, being composed of different genetic material.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable in this context, as it's not related to the phenomenon of two cell lines.
**Option B:** Mosaicism, while related to genetic variation, is not the same as chimerism, which involves the presence of two distinct cell lines from different zygotes.
**Option C:** Twinning is a related concept, but it's not the correct term for the presence of two cell lines within an individual.

**Clinical Pearl / High-Yield Fact**
Chimerism can be detected through genetic testing, such as DNA analysis, and has implications for organ transplantation, as the recipient's immune system may react against the donated cells.

**Correct Answer: C. Chimerism.