**Core Concept**
The underlying principle being tested is the assessment of fetal risk for Down's syndrome in a pregnant woman with a family history of the condition. This involves understanding the genetic basis of Down's syndrome and the role of screening tests in identifying high-risk pregnancies.

**Why the Correct Answer is Right**
The correct answer is **Non-invasive prenatal testing (NIPT)**, also known as cell-free DNA screening. This test involves analyzing the DNA present in the mother's blood to detect the presence of an extra copy of chromosome 21, which is associated with Down's syndrome. The test is highly sensitive and specific, with a detection rate of over 99% for Down's syndrome. The test works by detecting the presence of cell-free DNA fragments from the fetus in the mother's blood, which can indicate an increased risk of Down's syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** **Ultrasound Markers** - While ultrasound markers can be used to identify some fetal anomalies, they are not specific for Down's syndrome and are not recommended as a standalone screening test for this condition.

**Option B:** **Chorionic Villus Sampling (CVS)** - CVS is an invasive test that involves sampling cells from the placenta to diagnose chromosomal abnormalities, including Down's syndrome. However, it carries a risk of miscarriage and is typically not used for screening purposes.

**Option C:** **Maternal Serum Screening (MSS)** - MSS involves measuring the levels of certain substances in the mother's blood to estimate the risk of Down's syndrome. While MSS is a useful screening tool, it has a lower detection rate than NIPT and is often used in combination with other tests.

**Clinical Pearl / High-Yield Fact**
It's essential to note that NIPT is not diagnostic, and a positive result requires further testing, such as amniocentesis or CVS, to confirm the diagnosis.

**Correct Answer:** C.