Which structures are part of the upper respiratory system?

The upper respiratory system includes structures involved in the initial phases of breathing and is primarily responsible for filtering, warming, and humidifying the air we inhale. The oral cavity and nasal cavity are critical components of this system. The nasal cavity, lined with mucous membranes, plays a major role in air filtration, while the oral cavity also allows air to enter the respiratory tract.

In contrast, the trachea and alveoli (mentioned in the first option) are part of the lower respiratory system, with the trachea serving as the main airway to the lungs and the alveoli being the sites of gas exchange. The bronchi and bronchioles (the third option) also belong to the lower respiratory tract, branching from the trachea and leading into the lungs. Regarding the last option, the diaphragm is a muscle that aids in the process of breathing and is not a part of the respiratory passages themselves, while the larynx, though involved in voice production and protecting the airway, is still considered a lower respiratory structure. Thus, the correct categorization of the oral and nasal cavities as part of the upper respiratory system makes this option the answer.