**Core Concept**
The arch of the aorta is a critical structure in the thoracic cavity, playing a vital role in systemic blood circulation. It develops from the aortic sac, which is a dilated portion of the aortic root. During embryonic development, the aortic sac gives rise to the ascending aorta, the aortic arch, and the descending aorta.

**Why the Correct Answer is Right**
The correct answer is **B. Aortic sac**. The aortic sac is a precursor to the formation of the aortic arch, and it is from this structure that the arch of the aorta develops. As the embryo grows, the aortic sac undergoes a series of complex morphological changes, eventually giving rise to the aortic arch. This process is influenced by the action of various genetic and molecular factors, including transcription factors and signaling pathways.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the aortic valve is a separate structure that develops from the endocardial cushions, and it is not directly involved in the formation of the aortic arch.

**Option C:** This option is incorrect because the ductus arteriosus is a separate vascular structure that connects the pulmonary artery to the aortic arch, and it is not a precursor to the aortic arch itself.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the development of the aortic arch is a complex process that involves the coordinated action of multiple genetic and molecular factors. Abnormalities in this process can lead to congenital heart defects, such as aortic arch anomalies or coarctation of the aorta.

**Correct Answer: B. Aortic sac**