**Core Concept**
The right border of the heart is formed by the right atrium and the inferior vena cava, which are anatomically distinct but functionally related structures. The right atrium receives deoxygenated blood from the body through the superior and inferior vena cava, while the inferior vena cava is a major vein that carries deoxygenated blood from the lower body to the right atrium.

**Why the Correct Answer is Right**
The right border of the heart is primarily formed by the right atrium, which is the upper chamber that receives deoxygenated blood from the body. The right atrium is a thin-walled chamber that is divided into two parts: the smooth walled part (sinus venarum) and the rough walled part (caval part). The inferior vena cava, which is a major vein that carries deoxygenated blood from the lower body, also contributes to the formation of the right border of the heart. The inferior vena cava is a key landmark in cardiac anatomy and is often used as a reference point for identifying the right border of the heart.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the right atrium or the inferior vena cava, which are the primary structures that form the right border of the heart.

**Option B:** This option is incorrect because the left atrium is not a part of the right border of the heart. The left atrium is a separate chamber that receives oxygenated blood from the lungs.

**Option C:** This option is incorrect because the pulmonary trunk is not a part of the right border of the heart. The pulmonary trunk is a major artery that carries oxygenated blood from the right ventricle to the lungs.

**Option D:** This option is incorrect because the coronary sinus is not a part of the right border of the heart. The coronary sinus is a venous structure that collects deoxygenated blood from the myocardium and returns it to the right atrium.

**Clinical Pearl / High-Yield Fact**
The right border of the heart is an important landmark in cardiac anatomy, and the inferior vena cava is a key structure that can be used to identify the right border of the heart. The inferior vena cava is also a common site for cardiac catheterization and other invasive procedures.

**Correct Answer: C. Pulmonary trunk.**