**Core Concept**
The question is testing the knowledge of the anatomical characteristics of the cranial nerves, specifically their thickness. The cranial nerves are a group of 12 nerves that emerge directly from the brain and play crucial roles in various bodily functions, including sensation, movement, and autonomic functions.

**Why the Correct Answer is Right**
The cranial nerve that is thickest is the olfactory nerve (CN I). This is due to the presence of a large number of olfactory receptors in the olfactory epithelium, which are responsible for detecting odor molecules in the air. The olfactory nerve is also the shortest of the cranial nerves, measuring approximately 1-2 cm in length. Its thickness is a result of the compact arrangement of these receptors and the nerve fibers that transmit sensory information from the nose to the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** The optic nerve (CN II) is actually the thickest of the cranial nerves, not the olfactory nerve. It contains a large number of axons that transmit visual information from the retina to the brain.
**Option B:** The trigeminal nerve (CN V) is also a thick cranial nerve, but it is not the thickest. It is responsible for sensation in the face and motor functions such as chewing.
**Option C:** The facial nerve (CN VII) is a mixed nerve that contains both sensory and motor fibers. It is not the thickest cranial nerve.
**Option D:** The glossopharyngeal nerve (CN IX) is a mixed nerve that contains both sensory and motor fibers. It is not the thickest cranial nerve.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the cranial nerves can be remembered using the mnemonic "On Old Olympus' Towering Tops, A Finn And German Viewed Some Hops". Each letter in the sentence corresponds to the name of a cranial nerve, with the first letter of each word representing the first letter of the nerve's name.

**Correct Answer:** A. Optic nerve