Final answer:
The portion of the nasal cavity within the flexible tissues of the external nose is the vestibule, the correct answer is 'e. vestibule'. Nasal conchae are bony plates that warm and humidify incoming air but are not within the flexible tissues of the nose.
Step-by-step explanation:
The portion of the nasal cavity contained within the flexible tissues of the external nose is the vestibule. This area is where the nares (nostrils) open into the nasal cavity. The nasal septum is the wall composed of bone and cartilage that separates the left and right nasal cavities, but it is not the flexible portion within the external nose. Instead, nasal conchae are curved bony plates that project from the lateral walls of the nasal cavity and play a critical role in warming and humidifying the air that enters the nasal passages. They are not considered to be within the flexible tissues of the external nose either. The nasopharynx is part of the pharynx and not contained within the external nose. Therefore, the correct answer to the question is 'e. vestibule'.
The nasal conchae serve to increase the surface area of the nasal cavity and to disrupt the flow of air as it enters the nose, causing air to bounce along the epithelium where it is cleaned and warmed. The conchae and meatuses also help conserve water and prevent dehydration of the nasal epithelium by trapping water during exhalation.