**Core Concept**
The primary tumors of the heart are rare, but when they occur, they most commonly arise from the cardiac myocytes, which are the muscle cells of the heart. This type of tumor is known as a cardiac rhabdomyoma.

**Why the Correct Answer is Right**
Cardiac rhabdomyomas are the most common primary cardiac tumors in adults, particularly in the left ventricle. They are usually benign (non-cancerous) and can be associated with tuberous sclerosis complex, a genetic disorder. These tumors are composed of immature cardiac myocytes that have not fully differentiated.

**Why Each Wrong Option is Incorrect**
**Option A:** (Not specified) - This option is not provided, so we cannot evaluate its correctness.
**Option B:** Angiosarcoma - Angiosarcomas are malignant (cancerous) tumors of the heart, but they are not the most common primary cardiac tumors.
**Option C:** Fibroma - Cardiac fibromas are rare, benign tumors of the heart, but they are not as common as cardiac rhabdomyomas.

**Clinical Pearl / High-Yield Fact**
Cardiac rhabdomyomas are often associated with tuberous sclerosis complex, a genetic disorder characterized by the growth of non-cancerous tumors in various parts of the body, including the heart, brain, and skin.

**Correct Answer: C. Fibroma.**