**Core Concept**
The right border of the heart is formed by specific anatomical structures that can be visualized on a chest X-ray. The **right atrium** plays a crucial role in forming this border. Understanding the cardiac silhouette and its relationship with surrounding structures is essential for diagnosing cardiac conditions.

**Why the Correct Answer is Right**
The correct answer is related to the anatomical structure that forms the right border of the heart. The **right atrium** is indeed the primary structure that contributes to this border. This is because the right atrium is located on the right side of the heart and extends anteriorly, making it visible on a frontal chest X-ray.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately represent the primary structure forming the right border of the heart.
**Option B:** This option is also incorrect as it is not the primary contributor to the right heart border.
**Option C:** Similarly, this option is incorrect as it does not form the right border of the heart.

**Clinical Pearl / High-Yield Fact**
Remembering that the **right atrium** forms the right border of the heart is crucial for interpreting chest X-rays and diagnosing conditions such as cardiomegaly.

**Correct Answer:** D. Right Atrium