**Core Concept**
The sternum, also known as the breastbone, is a long, flat bone that forms the center of the chest. It plays a crucial role in protecting the heart and great vessels. The sternum is attached to the scapula through the costoclavicular ligament and the sternoclavicular joint, forming a crucial part of the thoracic cage.

**Why the Correct Answer is Right**
The correct answer involves the understanding of the anatomy of the thoracic cage. The sternum is attached to the scapula through the sternoclavicular joint and the costoclavicular ligament. This attachment provides stability to the scapula and allows for a wide range of movements. The sternoclavicular joint is a synovial joint that allows for flexion, extension, and rotation movements of the scapula.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not mention the sternoclavicular joint and the costoclavicular ligament, which are crucial for the attachment of the sternum to the scapula.
**Option B:** This option is incorrect as it mentions the clavicle but not the sternoclavicular joint and the costoclavicular ligament, which are essential for the attachment of the sternum to the scapula.
**Option C:** This option is incorrect as it mentions the rib cage but not the sternum or the scapula, which are the specific structures involved in this attachment.
**Option D:** This option is incorrect as it mentions the xiphoid process but not the sternoclavicular joint and the costoclavicular ligament, which are crucial for the attachment of the sternum to the scapula.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the sternoclavicular joint is a synovial joint that allows for a wide range of movements, including flexion, extension, and rotation. This joint is also a common site for injuries, such as sternoclavicular dislocation.

**Correct Answer: D. Xiphoid process**