Final answer:
During a mastectomy, care is essential to avoid damaging the long thoracic nerve, which can lead to issues like winged scapula and shoulder dysfunction. The correct answer is options 3.
Step-by-step explanation:
The long thoracic nerve is particularly at risk of injury during a mastectomy. This nerve innervates the serratus anterior muscle, which is responsible for the protraction and upward rotation of the scapula. During a mastectomy, typically for the treatment of breast cancer, care must be taken to avoid damaging this nerve to prevent postoperative complications like winged scapula, which can lead to impaired shoulder function.