**Core Concept**
The umbilical vesicle, also known as the yolk sac, is an early structure in embryonic development that provides nutrients to the embryo before the placenta forms. It plays a crucial role in the formation of the gastrointestinal tract and the development of the embryo's nutritional system.

**Why the Correct Answer is Right**
The umbilical vesicle attains full development around the fourth week of embryonic development. This is a critical period in the formation of the yolk sac, which then begins to degenerate as the placenta starts to form. The yolk sac is involved in the transfer of nutrients and waste products between the embryo and the maternal bloodstream. By the fourth week, the yolk sac has fully developed and begins to play a vital role in the embryo's nutrition.

**Why Each Wrong Option is Incorrect**
**Option A:** 2 week - At this stage, the embryo is still undergoing gastrulation, and the yolk sac is just beginning to form.
**Option C:** 6 week - By this time, the placenta has already started to form, and the yolk sac has begun to degenerate.
**Option D:** 7 week - The yolk sac's function has largely ceased by this stage, and the placenta is the primary source of nutrients for the embryo.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the development of the yolk sac is closely tied to the formation of the gastrointestinal tract. Abnormalities in yolk sac development can lead to gastrointestinal anomalies, such as omphalocele or gastroschisis.

**✓ Correct Answer: B. 4 week**