**Core Concept**
The non-stress test (NST) is a prenatal test used to assess fetal well-being by measuring the fetal heart rate in response to fetal movements. This test is based on the principle that a healthy fetus will increase its heart rate in response to movement, indicating normal fetal well-being.

**Why the Correct Answer is Right**
The non-stress test is a simple, non-invasive test that involves attaching two straps to the mother's abdomen, one above and one below the uterus, to measure the fetal heart rate. The test is considered "non-stress" because it does not cause any discomfort or stress to the mother or the fetus. The test is used to assess fetal well-being by measuring the fetal heart rate in response to fetal movements. A normal response is an increase in heart rate of 15 beats per minute or more within 30 seconds of fetal movement.

**Why Each Wrong Option is Incorrect**
**Option A:** Gestational age assessment typically involves ultrasound measurements, such as biparietal diameter, femur length, and abdominal circumference, to estimate fetal age.
**Option C:** Fetal arterial Doppler involves using ultrasound to measure the blood flow in the fetal arteries, which is used to assess fetal well-being and detect potential complications, such as growth restriction or placental insufficiency.
**Option D:** The progression of labor is typically assessed using a partograph, which is a graphical representation of the progress of labor, including cervical dilation, fetal heart rate, and other parameters.

**Clinical Pearl / High-Yield Fact**
The non-stress test is a useful tool for detecting fetal distress or compromise, but it is not a diagnostic tool for specific fetal conditions. A non-reactive non-stress test does not necessarily indicate fetal compromise, and further testing, such as a biophysical profile or an ultrasound examination, may be necessary to determine fetal well-being.

**✓ Correct Answer: B. Non-stress test**