**Core Concept**
The question tests knowledge of the venous drainage of the head and neck, specifically the formation of the **external jugular vein**. Understanding the tributaries and course of this vein is crucial for head and neck anatomy. The **retromandibular vein** and **posterior auricular vein** are key tributaries in this context.

**Why the Correct Answer is Right**
The **external jugular vein** is formed by the union of the posterior division of the **retromandibular vein** and the **posterior auricular vein**. This occurs in the neck, and the resulting vein then drains into the **subclavian vein**, facilitating venous return from the head and neck region. The **retromandibular vein** itself is formed by the maxillary and superficial temporal veins within the parotid gland.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not accurately describe the formation of the vein in question.
**Option B:** Incorrect as it misrepresents the tributaries involved.
**Option C:** Incorrect because it inaccurately describes the venous anatomy of the head and neck.

**Clinical Pearl / High-Yield Fact**
Remembering that the **external jugular vein** is a significant vein in the neck that can be used for central line placement or as a landmark for other surgical procedures is crucial. Its formation from the **retromandibular** and **posterior auricular veins** highlights the complex venous drainage of the head and neck.

**Correct Answer:** D. The external jugular vein.