**Core Concept**
The development of the heart involves the formation of the atrial septum, which is crucial for the separation of the left and right atria. The septum secundum is one of the two components of the atrial septum, the other being the foramen ovale.

**Why the Correct Answer is Right**
The septum secundum arises from the primitive atrium during embryonic development. Specifically, it develops from the right wall of the primitive atrium, growing towards the septum primum (the other component of the atrial septum). This process occurs around the 4th to 6th week of gestation. The septum secundum eventually forms a flap-like structure that covers the foramen ovale, a hole in the atrial septum that allows blood to flow between the left and right atria during fetal development.

**Why Each Wrong Option is Incorrect**
**Option A:** Bulbus cordis is a component of the embryonic heart that gives rise to the ventricles, but it is not involved in the formation of the septum secundum.
**Option B:** The primitive ventricle is a structure that forms during embryonic heart development, but it is not the source of the septum secundum.
**Option D:** The sinus venosus is a structure that contributes to the development of the right atrium, but it is not directly involved in the formation of the septum secundum.

**Clinical Pearl / High-Yield Fact**
Understanding the embryonic development of the atrial septum is crucial for recognizing congenital anomalies such as atrial septal defects (ASDs). An ASD is a hole in the atrial septum that can allow blood to flow between the left and right atria, potentially leading to complications such as heart failure and pulmonary hypertension.

✓ Correct Answer: C. Primitive atrium