**Core Concept**
The round ligament of the liver is a fibrous remnant of a fetal vessel that regresses during development. The ligament is formed from the obliterated umbilical vein, which plays a crucial role in the formation of the liver and its vascular connections.

**Why the Correct Answer is Right**
The umbilical vein is a vital structure that connects the fetus to the placenta, allowing for the exchange of oxygen, nutrients, and waste products. During fetal development, the umbilical vein regresses and forms the round ligament of the liver, which is a fibrous remnant of this vessel. The division of the round ligament during surgery would sever this remnant, indicating that the severed vessel is the umbilical vein.

**Why Each Wrong Option is Incorrect**
* **Option A:** The hepatic artery proper is not a fibrous remnant, but rather a vital vessel that supplies oxygenated blood to the liver.
* **Option B:** The portal vein is not a fibrous remnant, but rather a vital vessel that carries deoxygenated blood from the gastrointestinal tract to the liver.
* **Option D:** The hepatic portal vein tributaries are not a fibrous remnant, but rather a group of vessels that drain into the portal vein.

**Clinical Pearl / High-Yield Fact**
The round ligament of the liver is an important landmark during liver surgery, and its division can lead to bleeding from the umbilical vein remnant. It is essential to identify and ligate this vessel during surgery to prevent postoperative bleeding.

**Correct Answer:** C.