Which of the following structures is primarily responsible for carrying air to the lungs?

The trachea, commonly referred to as the windpipe, serves as the main conduit for air traveling to and from the lungs. It extends from the larynx and branches into the bronchi, which enter the lungs. By providing a clear passage for air, the trachea ensures that oxygen can reach the bronchi and subsequently the bronchioles, leading to the alveoli where gas exchange occurs.

The alveoli are tiny air sacs in the lungs where oxygen and carbon dioxide exchange happens; they do not carry air but rather facilitate this essential exchange. Bronchioles, which branch off from the bronchi, are smaller air passages leading to the alveoli, but they are not the primary structure for transporting air into the lungs as the trachea is. The diaphragm is a muscle that plays a critical role in ventilation by contracting and relaxing to create negative pressure in the thoracic cavity, allowing air to flow into the lungs, but it does not carry air itself.

Considering the role of each structure, the trachea is the primary pathway for directing air into the lungs, making it the correct answer.