Final answer:
The mouth does not need to be blocked when food moves from the pharynx into the esophagus because it is not an air passage to the respiratory tract, unlike the nasopharynx and larynx which are sealed off during swallowing. the correct option is (b).
Step-by-step explanation:
The pharynx is a key anatomical feature in the human body that serves dual roles in respiratory and digestive systems. When food is swallowed, certain openings must be closed to ensure that food does not enter the respiratory tract. The correct answer to the question of which opening does not need to be blocked when food moves from the pharynx into the esophagus is d. mouth. This is because the mouth is not an air passage that leads to the respiratory tract; it is just the entrance to the digestive system.
Although the nasopharynx, oropharynx, and larynx are all part of the pharynx or connected to it, the nasopharynx is involved only in breathing and speech and is automatically sealed off by the soft palate and uvula during swallowing to prevent food from entering the nasal cavity. The oropharynx is a passage for both air and food and therefore needs to be open for food to pass through. The larynx, which would allow food to go down the wrong pipe into the trachea, is guarded by the epiglottis that covers the glottis during swallowing, effectively preventing the entrance of food into the respiratory tract.