**Core Concept**
The ligamentum teres is a fibrous remnant of a major vessel that played a crucial role in fetal circulation. In the fetal circulation, oxygenated blood from the placenta was transported to the fetus via the umbilical vein. After birth, this vessel undergoes significant changes, and its remnants form the ligamentum teres.

**Why the Correct Answer is Right**
The correct answer, B. Remnant of umbilical vein, is supported by the fact that the ligamentum teres is formed from the obliterated umbilical vein, which was responsible for carrying oxygenated blood from the placenta to the fetus. This process occurs after birth, when the umbilical vein loses its function and eventually becomes fibrotic, forming the ligamentum teres. The obliteration of the umbilical vein is a result of the decrease in blood flow and pressure after birth.

**Why Each Wrong Option is Incorrect**
**Option A:** The ductus arteriosus is a remnant that connects the pulmonary artery to the aortic arch, allowing blood to bypass the lungs in the fetus. It does not form the ligamentum teres.
**Option C:** The ductus venosus is a shunt that connects the umbilical vein to the inferior vena cava, allowing oxygenated blood to bypass the liver in the fetus. It also does not form the ligamentum teres.
**Option D:** The umbilical arteries carry deoxygenated blood from the fetus to the placenta and do not form the ligamentum teres.

**Clinical Pearl / High-Yield Fact**
The ligamentum teres can be a useful landmark in abdominal surgery, particularly during laparoscopic procedures. It can be used to locate the umbilicus and the surrounding structures.

**✓ Correct Answer: B. Remnant of umbilical vein**