**Core Concept**
The question tests knowledge of the **cranial nerves**, specifically their anatomical characteristics. The **twelfth cranial nerve**, also known as the **hypoglossal nerve**, plays a crucial role in tongue movement.

**Why the Correct Answer is Right**
The correct answer is related to the **twelfth cranial nerve**, which is indeed the **smallest cranial nerve** in terms of the number of fibers it contains. This nerve is primarily responsible for **motor innervation** of the tongue, controlling its movements.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not correspond to the smallest cranial nerve.
**Option B:** Similarly, this option is also incorrect as it is not the smallest cranial nerve.
**Option D:** This choice is incorrect as well, not being the smallest cranial nerve.

**Clinical Pearl / High-Yield Fact**
The **hypoglossal nerve** is a key structure in **neurological examinations**, as its dysfunction can lead to **tongue weakness** or **deviation**.

**Correct Answer:** D. Twelfth cranial nerve