**Core Concept**
The child's presentation suggests a musculoskeletal issue, specifically involving the cervical spine and possibly the thoracic outlet. The restricted range of motion and pain in the neck and right shoulder point towards a condition that affects the nerves or blood vessels in this region.

**Why the Correct Answer is Right**
The most likely diagnosis is a **Torticollis**, also known as wry neck. This condition is characterized by an abnormal, twisted posture of the neck, often due to a **sternocleidomastoid muscle spasms** or **fibromyalgia**. The spasms can cause pain and stiffness in the neck and shoulder region, leading to restricted mobility.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Cervical Radiculopathy** is a condition that affects the nerve roots in the cervical spine, causing pain, numbness, or weakness in the arm. While it can cause pain and restricted mobility, it typically presents with more severe neurological symptoms, such as muscle weakness or sensory deficits.
* **Option B:** **Thoracic Outlet Syndrome** is a condition that affects the nerves or blood vessels in the thoracic outlet, causing pain and numbness in the arm. However, it typically presents with symptoms in the arm, rather than the neck and shoulder.
* **Option C:** **Costochondritis** is an inflammation of the cartilage that connects the ribs to the breastbone, causing pain in the chest and shoulder region. While it can cause pain in the shoulder, it is not typically associated with restricted range of motion or neck pain.

**Clinical Pearl / High-Yield Fact**
Torticollis can be caused by a variety of factors, including muscle spasms, fibromyalgia, or even viral infections. It's essential to perform a thorough physical examination to rule out other conditions that may mimic torticollis.

**Correct Answer: C. Torticollis**