**Core Concept**
Down's Syndrome is a genetic disorder caused by an extra copy of chromosome 21, leading to intellectual disability and characteristic physical features. The condition is diagnosed prenatally using specific markers in maternal serum and fetal ultrasound.

**Why the Correct Answer is Right**
The correct answer is a marker for Down's Syndrome because it is a protein produced by the placenta in response to the increased demand for fetal growth and development. Elevated levels of human chorionic gonadotropin (hCG) and alpha-fetoprotein (AFP) are often associated with Down's Syndrome. The combination of high hCG and low AFP levels is a specific marker for Down's Syndrome, as it suggests a higher risk of the fetus having an extra chromosome 21.

* **Option A:** This option is not correct because it is not a marker for Down's Syndrome. It may be a marker for other conditions, but not Down's Syndrome.
* **Option B:** This option is not correct because it is a marker for other conditions, such as multiple pregnancy or fetal anemia. It may be elevated in Down's Syndrome, but it is not a specific marker for the condition.
* **Option C:** This option is not correct because it is not a marker for Down's Syndrome. It may be a marker for other conditions, such as thyroid disease or liver disease.

**Clinical Pearl / High-Yield Fact**
The quad screen test, which measures hCG, AFP, estriol, and inhibin A, is a commonly used screening test for Down's Syndrome. However, it is essential to note that a high-risk result does not necessarily mean that the fetus has Down's Syndrome, as many false positives occur.

**Correct Answer: D. Human chorionic gonadotropin (hCG)**