**Core Concept**
The posterior triangle of the neck is an anatomical region bounded by the sternocleidomastoid muscle anteriorly, the trapezius muscle posteriorly, and the base of the skull superiorly. It contains several important structures, including nerves, muscles, and lymph nodes.

**Why the Correct Answer is Right**
The posterior triangle is divided into two parts by the inferior belly of the omohyoid muscle. The upper part is known as the occipital triangle, which contains the greater occipital nerve and the suboccipital muscles. The lower part is known as the supraclavicular triangle, which contains the transverse cervical artery and the suprascapular nerve. The correct contents of the posterior triangle include the accessory nerve (XI), the subclavian artery, the subclavian vein, the transverse cervical artery, the suprascapular artery, the omohyoid muscle, and the trapezius muscle.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the accessory nerve (XI) is actually a content of the posterior triangle of the neck.
**Option B:** This option is incorrect as the subclavian artery is actually a content of the posterior triangle of the neck.
**Option C:** This option is incorrect as the omohyoid muscle is actually a content of the posterior triangle of the neck.

**Clinical Pearl / High-Yield Fact**
The posterior triangle is an important region for clinical examination, particularly for the assessment of the accessory nerve (XI) and the subclavian artery.

**Correct Answer: A**
Accessory nerve (XI) is a content of the posterior triangle of the neck.