Final answer:
The soft palate is the structure that divides the rostral part of the pharynx into the oropharynx and nasopharynx, serving as a divider that prevents ingested materials from entering the nasal cavity during swallowing. Option a.
Step-by-step explanation:
The structure that divides the rostral part of the pharynx into the oropharynx and nasopharynx is the soft palate. The oropharynx is a passageway for both air and food and is bordered superiorly by the nasopharynx. The soft palate, along with the uvula, rises reflexively to close off the nasopharynx during swallowing, preventing ingested material from entering the nasal cavity. This action highlights its role as a functional divider between the oropharynx and nasopharynx. Therefore, the correct answer to the question is 'b) Soft palate'.