Which of the following structures does not have cilia?

The alveoli do not have cilia, and this is the reason why they are the correct answer. Alveoli are tiny, balloon-like structures in the lungs where gas exchange occurs. Their primary function is to facilitate the diffusion of oxygen and carbon dioxide between the air and blood. The walls of the alveoli consist of a single layer of epithelial cells (type I and type II alveolar cells) that allow for efficient gas exchange.

Cilia are hair-like structures that help to move mucus and trapped particles out of the respiratory tract, which is particularly important in the upper respiratory passages like the trachea and bronchi. In contrast, alveoli do not require cilia because their function does not involve clearing debris or mucus in the same way; they are designed specifically for gas exchange.

Because the other structures listed—trachea, bronchi, and respiratory epithelium—are all part of the conducting zone of the respiratory system, they contain ciliated cells to help protect the lungs from pathogens and particles.