Final answer:
The trachea conducts air to the lungs, branching into bronchi and bronchioles that terminate in alveoli, the primary sites for gas exchange. Notably, the diaphragm and intercostal muscles aid respiration, while the nasal cavity and pharynx belong to the upper respiratory system.
Step-by-step explanation:
The trachea, or windpipe, is a tube that carries air to the lungs where it divides into the left and right bronchi. These then branch into smaller tubes called bronchioles which end in alveoli, tiny sacs where gas exchange occurs. The diaphragm and intercostal muscles aid in respiration but are not part of the lower respiratory system itself, and the nasal cavity and pharynx are part of the upper respiratory system. The correct answer to the question is b) Lungs and alveoli, which are the primary sites for gas exchange in the respiratory system.