**Core Concept**
Chimerism is a phenomenon where two or more cell lines from different zygotes coexist in a single individual. This can occur due to various reasons such as twin-to-twin transfusion, bone marrow transplantation, or even in vitro fertilization.

**Why the Correct Answer is Right**
The correct answer refers to a person showing two cell lines derived from two different zygotes. This is known as chimerism, where the presence of two distinct cell populations can be identified through genetic analysis. Chimerism can occur due to the fusion of two fertilized eggs (zygotes) or the exchange of cells between fetuses in the womb, resulting in the coexistence of two genetically distinct cell lines.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it refers to a condition where a person has two sets of DNA, one from each parent, but does not account for the presence of two distinct cell lines from different zygotes.
* **Option B:** This option is incorrect as it refers to a condition where a person has a mixture of cells from two different individuals, but does not necessarily imply the presence of two distinct cell lines from different zygotes.
* **Option C:** This option is incorrect as it refers to a condition where a person has a mixture of cells from two different tissues, but does not account for the presence of two distinct cell lines from different zygotes.

**Clinical Pearl / High-Yield Fact**
Chimerism can have significant implications in forensic medicine, where it can be used to identify individuals or resolve paternity disputes.

**Correct Answer: C. Chimerism.**