**Core Concept**
Non-stress test (NST) is a widely used method to assess fetal well-being, particularly in the third trimester. It involves monitoring the fetal heart rate in response to fetal movements, providing valuable insight into the fetus's autonomic nervous system function.

**Why the Correct Answer is Right**
The correct answer is a non-invasive and reliable method to evaluate fetal well-being. The NST measures the fetal heart rate in response to fetal movements, known as accelerations, which are a sign of a healthy fetus. Accelerations are caused by the release of catecholamines, such as epinephrine and norepinephrine, in response to fetal movements. These hormones stimulate the sinoatrial node, leading to an increase in fetal heart rate. The presence of two or more accelerations in a 20-minute period is considered a normal result, indicating fetal well-being.

**Why Each Wrong Option is Incorrect**
* **Option A:** Contraction stress test (CST) is a more invasive method that assesses fetal well-being by monitoring the fetal heart rate response to uterine contractions. While it can provide valuable information, it is not the single best parameter to assess fetal well-being.
* **Option B:** Biophysical profile (BPP) is a comprehensive assessment of fetal well-being that includes four components: fetal tone, breathing movements, movement, and amniotic fluid volume. While it is a valuable tool, it is more complex and time-consuming than the NST.
* **Option C:** Fetal ultrasound is a valuable tool for assessing fetal anatomy and growth, but it does not directly assess fetal well-being.

**Clinical Pearl / High-Yield Fact**
A non-stress test is a quick and easy way to assess fetal well-being, and it should be performed at least twice a week in high-risk pregnancies.

**Correct Answer:** B. Biophysical profile (BPP)