**Core Concept**
The umbilical cord is a vital structure connecting the fetus to the placenta, facilitating the exchange of oxygen, nutrients, and waste products. It is composed of two arteries and one vein, which are embedded in a gelatinous substance called Wharton's jelly.

**Why the Correct Answer is Right**
The umbilical cord contains two umbilical arteries, which carry deoxygenated blood from the fetus to the placenta for gas exchange. The two arteries are responsible for transporting waste products, such as carbon dioxide and urea, from the fetus to the placenta. In contrast, the single umbilical vein carries oxygenated, nutrient-rich blood from the placenta to the fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the umbilical cord contains only two arteries, not three. The presence of three arteries is not a normal anatomical feature of the umbilical cord.
**Option B:** This option is incorrect because the umbilical cord contains only one vein, not two. The single umbilical vein is responsible for transporting oxygenated blood from the placenta to the fetus.
**Option C:** This option is incorrect because the umbilical cord does not contain any lymphatic vessels. The umbilical cord is primarily composed of blood vessels and Wharton's jelly.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the umbilical cord is a dynamic structure that changes in diameter and length throughout fetal development. The cord's length and diameter can affect fetal well-being, and abnormalities in cord length or diameter may indicate fetal distress or other complications.

**Correct Answer: D. Two arteries and one vein.**