Final answer:
The action of the epiglottis folding down prevents the bolus from entering the trachea during swallowing, ensuring that food passes safely from the pharynx to the esophagus.
Step-by-step explanation:
The involuntary movement that prevents the bolus from entering the trachea is the action of the epiglottis folding down and covering the trachea. When the larynx is pulled superiorly during swallowing, the epiglottis, which is part of the larynx, folds inferiorly to cover the glottis, thus blocking the opening to the trachea and preventing food or liquid from entering the respiratory tract. This action is crucial for ensuring that the bolus safely moves from the pharynx to the esophagus without entering the larynx, which could lead to choking or aspiration.
The involuntary movement that prevents the bolus from entering the trachea is option D: The epiglottis folds down and covers the trachea. During swallowing, the tongue moves backward, causing the epiglottis to cover the opening of the larynx, which blocks access to the trachea and prevents food from entering it. This reflexive movement helps ensure that food passes from the pharynx to the esophagus and not into the respiratory tract.