**Core Concept**
The question tests the student's knowledge of the cardiac anatomy, specifically the structures forming the right border of the heart. The right border of the heart is formed by several structures, including the superior vena cava, the right atrium, and the inferior vena cava.

**Why the Correct Answer is Right**
The correct answer is the right atrium. The right atrium is one of the four chambers of the heart and is the upper chamber on the right side of the heart. It receives deoxygenated blood from the body through the superior and inferior vena cavae and pumps it into the right ventricle. The right atrium is a key structure in the cardiac anatomy and is easily palpable on the right border of the heart.

**Why Each Wrong Option is Incorrect**
* **Option A:** SVC (Superior Vena Cava) does contribute to the right border of the heart, but it is not the primary structure forming the right border. SVC forms the upper part of the right border.
* **Option B:** IVC (Inferior Vena Cava) also contributes to the right border of the heart, but it is not the primary structure forming the right border. IVC forms the lower part of the right border.
* **Option D:** Lt. atrium appendage is not a correct option as it is located on the left side of the heart and not on the right border.
* **Option E:** Pulmonary vessels do not contribute to the right border of the heart. They are located in the left side of the mediastinum and arise from the right ventricle.

**Clinical Pearl / High-Yield Fact**
The right border of the heart is formed by the right atrium, which is a key structure in the cardiac anatomy. The right atrium is a site for various cardiac procedures, including cardiac catheterization and pacemaker insertion.

**Correct Answer: C. Right atrium**