Final answer:
The sternum is the structure that protects key respiratory organs and mediastinal structures such as the heart and vessels, making it the correct answer to the question.
Step-by-step explanation:
The structure that protects important organs of the respiratory system and vital structures within the mediastinum, such as the heart and great vessels, is the sternum. The thoracic cavity involves several components including the superior mediastinum, pleural cavity, pericardial cavity within the mediastinum, and is bounded at the lower end by the diaphragm. The thoracic cage, or rib cage, is comprised of the ribs, sternum, thoracic vertebrae, and costal cartilages. This cage safeguards the thoracic cavity's organs, especially the heart and lungs. Therefore, option (a) Sternum is the correct answer.