**Core Concept**
The question is testing knowledge of the most common type of benign cardiac tumor that occurs within the heart's cavities. Cardiac tumors can be benign or malignant and arise from various cardiac tissues, including the myocardium, endocardium, and pericardium.

**Why the Correct Answer is Right**
The correct answer is the myxoma, which is a type of benign cardiac tumor that primarily affects the left atrium. Myxomas are known to arise from the interatrial septum, often near the fossa ovalis. They are usually solitary, pedunculated tumors that can cause obstructive symptoms due to their size or location, leading to signs such as dyspnea, palpitations, or even syncope. The pathogenesis of myxomas is not fully understood, but they are thought to arise from mesenchymal cells within the heart.

**Why Each Wrong Option is Incorrect**
**Option A:** Rhabdomyoma is a benign cardiac tumor, but it is more common in children and typically affects the ventricles, not the intracavitary space.
**Option B:** Fibroelastoma is a rare type of cardiac tumor that can occur on the valve leaflets or the endocardium but is not the most common intracavitary benign tumor.
**Option C:** Lipoma is a type of benign tumor that can occur in the heart, but it is not as common as myxoma and typically affects the epicardium or the myocardium.

**Clinical Pearl / High-Yield Fact**
Myxomas are the most common primary cardiac tumor in adults and are more common in women, especially those of middle age. They can also occur in association with certain syndromes, such as Carney complex.

**Correct Answer:** D. Myxoma.