**Core Concept**
The question tests knowledge of thoracic anatomy, specifically the structures located posterior to the sternum. The sternum, also known as the breastbone, is a long, flat bone in the center of the chest.

**Why the Correct Answer is Right**
The correct answer is related to the mediastinum, which contains the heart, trachea, esophagus, and other vital structures. Posterior to the sternum, these structures are indeed found within the mediastinum.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it's not directly related to the anatomical space posterior to the sternum.
**Option B:** Incorrect as it refers to a different anatomical location.
**Option C:** Incorrect because, although part of the thoracic cavity, it's not the most specific answer to what lies directly posterior to the sternum.
**Option D:** Incorrect as it does not accurately describe the anatomical relationship.

**Clinical Pearl / High-Yield Fact**
Remember, the mediastinum is divided into anterior, middle, and posterior parts, each containing different structures. This distinction is crucial for diagnosing and treating conditions affecting these areas.

**Correct Answer:** Correct Answer: C. Mediastinum