Final answer:
The soft palate and uvula are the structures that fall back and can block the posterior pharynx when the mandible relaxes, particularly during unconsciousness or compromised muscle control.
Step-by-step explanation:
The structure with the tendency to fall back and occlude the posterior pharynx when the mandible relaxes is the soft palate and the uvula. Usually during swallowing, the soft palate and uvula rise reflexively to close off the entrance to the nasopharynx. Simultaneously, the larynx is pulled up, and the epiglottis folds down, covering the glottis, which blocks access to the trachea and prevents food from entering the airways. If these reflexes fail, the soft palate and uvula may not rise adequately, which can lead to the obstruction of the posterior pharynx, a situation that may occur when one is unconscious or muscle control is compromised.