**Core Concept**
Functional radical dissection of the neck is a surgical procedure that aims to remove cancerous tissues while preserving vital structures. This procedure involves the removal of the primary tumor, surrounding lymph nodes, and other affected tissues while leaving intact essential structures necessary for maintaining neck function and preventing long-term morbidity.

**Why the Correct Answer is Right**
In a functional radical dissection of the neck, the sternomastoid muscle (b) and the accessory nerve (d) are preserved to maintain neck mobility and prevent shoulder drooping (winging). The internal jugular vein (a) is preserved to maintain venous drainage of the brain and prevent complications such as venous hypertension. The preservation of these structures is crucial for postoperative recovery and quality of life.

**Why Each Wrong Option is Incorrect**
**Option A:** The internal carotid artery is not preserved in a functional radical dissection of the neck, as it may be involved in the tumor or adjacent to it, and its removal is often necessary for oncological control.

**Option B:** The lymph nodes (c) are typically removed in a radical neck dissection to clear the tumor and prevent recurrence. While some lymph nodes may be preserved in a functional neck dissection, the primary goal is to remove the tumor-bearing lymph nodes.

**Option C:** There is no need to preserve the sternocleidomastoid muscle's counterpart, the trapezius muscle, in a functional radical dissection of the neck.

**Clinical Pearl / High-Yield Fact**
In neck dissection, the order of the spinal accessory nerve branches (cranial root and spinal root) is crucial for preserving shoulder function. The cranial root supplies the sternocleidomastoid muscle, while the spinal root innervates the trapezius muscle.

**✓ Correct Answer: C. abd. Sternomastoid muscle and accessory nerve are preserved in a functional radical dissection of the neck.**