**Core Concept**
Radical neck dissection is a surgical procedure that involves the removal of lymph nodes and other structures in the neck to treat or prevent the spread of cancer. This procedure aims to eliminate potential sites of metastasis while preserving essential functions such as swallowing, speech, and facial movements.

**Why the Correct Answer is Right**
The primary goal of preserving structures during radical neck dissection is to maintain the continuity of the facial nerve, which controls the muscles of facial expression. The surgeon aims to spare the spinal accessory nerve, which is responsible for shoulder movement, and the sternocleidomastoid muscle, which helps in stabilizing the head. The preservation of these structures enables the patient to maintain function and mobility in the neck and shoulder region.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the internal jugular vein is often sacrificed during radical neck dissection to ensure complete removal of lymph nodes and to prevent the spread of cancer.
* **Option B:** This option is incorrect because the carotid artery is sometimes sacrificed during radical neck dissection, especially if it is involved with cancer. However, the surgeon will attempt to preserve it whenever possible.
* **Option C:** This option is incorrect because the phrenic nerve is not directly involved in radical neck dissection. The phrenic nerve is responsible for controlling the diaphragm, and its preservation is not a primary concern in this procedure.

**Clinical Pearl / High-Yield Fact**
It is essential for surgeons to carefully identify and preserve the spinal accessory nerve and the sternocleidomastoid muscle during radical neck dissection to prevent long-term complications such as shoulder dysfunction and facial asymmetry.

**Correct Answer:** D. Sternocleidomastoid muscle, Spinal Accessory nerve.