**Core Concept**
The best surgical approach for accessing a retrosternal goitre involves consideration of the anatomy of the thyroid gland, the proximity of the goitre to vital structures, and the need for optimal exposure and visualization. The key principle is to achieve a balance between minimizing the risk of injury to surrounding structures and ensuring adequate exposure for safe resection.

**Why the Correct Answer is Right**
The correct surgical approach for a retrosternal goitre is a collar incision or a horizontal incision in the neck, often extending into the sternal notch. This approach allows for optimal exposure of the retrosternal goitre, enabling the surgeon to safely resect the gland while minimizing the risk of injury to the trachea, esophagus, and recurrent laryngeal nerves. The incision is typically made in the midline, and the strap muscles are divided to expose the trachea and the retrosternal goitre.

**Why Each Wrong Option is Incorrect**
**Option A:** is not a suitable approach for accessing a retrosternal goitre, as it may not provide adequate exposure and may increase the risk of injury to surrounding structures.
**Option B:** is not a standard approach for thyroid surgery, and it may not provide the necessary exposure for resecting a retrosternal goitre.
**Option C:** may not be sufficient for accessing a retrosternal goitre, as it may not provide adequate exposure of the gland and surrounding structures.

**Clinical Pearl / High-Yield Fact**
When planning the surgical approach for a retrosternal goitre, it is essential to consider the size and location of the goitre, as well as the patient's overall medical condition. A thorough preoperative evaluation, including imaging studies and endoscopy, can help determine the best approach and minimize the risk of complications.

**Correct Answer: C. Horizontal incision extending into the sternal notch.**