**Core Concept**
The sub-occipital triangle is a significant anatomical landmark in the posterior cranial fossa, bounded by the rectus capitis posterior major muscle, the superior oblique muscle, and the inferior oblique muscle. This region is crucial for the passage of vital structures, including blood vessels and nerves, that supply the cranium and the upper cervical spine.

**Why the Correct Answer is Right**
The occipital artery primarily supplies the scalp and is not a content of the sub-occipital triangle. In contrast, the vertebral artery, which is a key structure within the triangle, ascends through the neck and enters the cranial cavity through the foramen magnum. The dorsal ramus of C1 nerve, also known as the suboccipital nerve, is a branch of the C1 spinal nerve that pierces the suboccipital triangle. The suboccipital plexus of veins is a network of veins that drains the region, including the vertebral veins and the occipital veins.

**Why Each Wrong Option is Incorrect**
**Option A:** The vertebral artery is indeed a content of the sub-occipital triangle, making this option incorrect.
**Option B:** The dorsal ramus of C1 nerve is also a content of the sub-occipital triangle, so this option is incorrect.
**Option C:** The suboccipital plexus of vein is a content of the sub-occipital triangle, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
The sub-occipital triangle is a critical region for the surgical approach to the craniocervical junction, particularly for procedures involving the atlanto-axial joint or the vertebral artery.

**✓ Correct Answer: D. Occipital artery**