**Core Concept**
The subclavian artery is a major blood vessel that supplies oxygenated blood to the arms. Stenosis refers to the narrowing of the artery's lumen, which can lead to reduced blood flow and ischemia. Understanding the anatomy and common sites of stenosis is crucial for diagnosing and managing vascular diseases.

**Why the Correct Answer is Right**
The commonest part of the subclavian artery to be affected by stenosis is the left subclavian artery at the site of the **costoclavicular junction**. This is because the left subclavian artery is more prone to compression by the clavicle, leading to stenosis. The compression is thought to be due to the higher incidence of left-sided aortic arches, which can cause the left subclavian artery to be more tortuous and susceptible to compression.

**Why Each Wrong Option is Incorrect**
* **Option A:** The right subclavian artery is not the most common site of stenosis, although it can be affected due to similar causes of compression.
* **Option B:** The vertebral artery is a branch of the subclavian artery, but it is not the part of the subclavian artery that is most commonly affected by stenosis.
* **Option D:** The aortic arch is the origin of the subclavian artery, but it is not the part of the subclavian artery that is most commonly affected by stenosis.

**Clinical Pearl / High-Yield Fact**
The costoclavicular junction is a common site of compression for the left subclavian artery, leading to stenosis. This is often due to a higher incidence of left-sided aortic arches, which can cause the left subclavian artery to be more tortuous and susceptible to compression.

**Correct Answer: C. Left subclavian artery at the costoclavicular junction.**