**Core Concept**
The treatment of choice for an impacted fracture of the surgical neck of the humerus involves stabilizing the fracture to ensure proper healing and prevent complications such as nonunion or malunion. This type of fracture requires a surgical approach to achieve optimal outcomes.

**Why the Correct Answer is Right**
Impacted fractures of the surgical neck of the humerus are typically treated with a shoulder arthroplasty or hemiarthroplasty, which involves replacing the damaged joint with an artificial one. This is because the fracture is often unstable and may not heal properly on its own, and the patient may have limited mobility and function in the affected arm. The surgery aims to restore the patient's range of motion and alleviate pain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a closed reduction and percutaneous pinning may not be sufficient to stabilize the fracture, especially in an impacted fracture where the bone fragments are tightly compressed.

**Option B:** This option is incorrect because a shoulder arthroscopy is not the primary treatment for an impacted fracture of the surgical neck of the humerus, although it may be used as an adjunct to assess and treat any associated labral or rotator cuff injuries.

**Option C:** This option is incorrect because a cast or sling may not provide sufficient stabilization for an impacted fracture of the surgical neck of the humerus, and may lead to complications such as nonunion or malunion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that impacted fractures of the surgical neck of the humerus often require surgical intervention to prevent long-term complications and ensure optimal outcomes.

**Correct Answer:** C. Hemiarthroplasty.