**Core Concept**
The assessment of fetal well-being during pregnancy is crucial for preventing adverse outcomes. Non-stress testing (NST) and biophysical profile (BPP) are two commonly used methods to evaluate fetal health.

**Why the Correct Answer is Right**
The biophysical profile (BPP) is a comprehensive test that assesses fetal well-being by evaluating five parameters: fetal tone, breathing movements, gross body movements, fluid volume (amniotic fluid index), and non-stress test (NST). The BPP provides a more detailed assessment of fetal health compared to the non-stress test alone, making it a more sensitive test for evaluating fetal well-being.

**Why Each Wrong Option is Incorrect**
* **Option A:** The non-stress test (NST) is a less sensitive test compared to the biophysical profile (BPP) and is often used as a screening tool. It measures fetal heart rate in response to fetal movement but does not provide a comprehensive evaluation of fetal well-being.
* **Option B:** The Doppler ultrasound is used to assess fetal well-being by measuring blood flow through the umbilical artery. While it is a useful tool, it is not as comprehensive as the biophysical profile (BPP) and is often used in conjunction with other tests.
* **Option C:** The maternal-fetal medicine (MFM) unit is a specialized department that provides care for high-risk pregnancies. However, it is not a test for assessing fetal well-being.

**Clinical Pearl / High-Yield Fact**
The biophysical profile (BPP) is a useful test for assessing fetal well-being, but it should be used in conjunction with other tests to provide a comprehensive evaluation of fetal health.

**Correct Answer:** B.