Final answer:
The esophagus is the structure that lies just posterior to the trachea; it is responsible for transporting food from the pharynx to the stomach and enters the abdomen through the esophageal hiatus.
Step-by-step explanation:
The structure that lies just posterior to the trachea is the esophagus. The esophagus is a muscular tube responsible for conveying food from the pharynx to the stomach. In terms of anatomical position, the esophagus is located behind the trachea and remains in a collapsed form when not in use, that is, when not engaged in swallowing. Its entry into the abdomen is through an opening in the diaphragm called the esophageal hiatus. This is distinct from the thyroid gland, which is situated anterior to the trachea and inferior to the larynx. In contrast, the larynx is located superior to the trachea, and it is the structure through which the air passes on its way to and from the lungs.