**Core Concept**
The question is testing the visibility of cranial nerves on imaging modalities, specifically plain CT scans. The cranial nerves are a set of 12 paired nerves that arise 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 can be visualized on plain CT scans is the **Optic Nerve (CN II)**. This is because the optic nerve is a large, fatty, and well-myelinated nerve that can be easily visualized on CT scans due to its high density. The optic nerve is also relatively superficial and can be seen as a distinct structure on CT scans, making it the most visible cranial nerve on this imaging modality.

**Why Each Wrong Option is Incorrect**
**Option A:** The Oculomotor nerve (CN III) is not easily visible on plain CT scans due to its smaller size and deeper location within the brainstem.
**Option B:** The Trochlear nerve (CN IV) is also not visible on plain CT scans due to its small size and deep location within the brainstem.
**Option C:** The Abducens nerve (CN VI) is not visible on plain CT scans due to its small size and deep location within the brainstem.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the optic nerve is the only cranial nerve that can be visualized on plain CT scans, making it a useful landmark for identifying other structures in the orbit.

**Correct Answer:** . Optic Nerve (CN II)