The correct answer would be B, the opening of the parotid duct. This sounds very scientific, so in lay terms, it’s the spit glands. A vestibule is basically another opening within an opening - think a hallway with multiple hallways coming off it without doors; each of those other halls would be a vestibule within the original hall.
So, the parotid duct opens in the mouth to get the spit into the mouth as you chew, or breathe, or just go about your life. After all, saliva is one of the first steps in the digestion process, so this has to happen as you’re eating.
If that was too long an explanation for you, the opening of the parotid duct is the answer because it brings saliva from the salivary glands to the mouth.
Opening of the parotid duct- the oral vestibule is the space in the mouth lying between the lips and the teeth. see netter plate 45 for an illustration of the mouth and vestibule-although the vestibule isnt labeled, you can get some idea of where it lies. you can also see that the parotid duct is opening into this space . the parotid duct drains the parotid gland; it crosses the masseter to enter the cheek and drain into the oral cavity. the parotid duct drains into the cheek near the upper 2nd molar tooth.
the tongue is not in the vestibule-its behind the teeth. the submandibular duct opens into the area under the tongue, near the tongues frenulum. the sublingual fold is also underneath the tongue; this is the fold that contains the openings for the ducts of the sublingual gland to drain into the mouth. the uvula is the punching bag hanging from the soft palate at the back of the oral cavity.