The auriculotemporal nerve, which carries postganglionic parasympathetic fibers to the parotid gland, is a branch of which division of the trigeminal nerve?

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The important point is that the parotid gland’s secretomotor (parasympathetic) fibers travel along a branch of the trigeminal nerve that arises from the lower division, not the upper ones. The auriculotemporal nerve is a branch of the mandibular division of the trigeminal nerve (the lower division, V3). This division provides sensory innervation to the lower face and, importantly, gives off the auriculotemporal nerve, which serves as the conduit for the postganglionic parasympathetic fibers to reach the parotid gland.

Those parasympathetic fibers originate in the glossopharyngeal pathway (via the otic ganglion) and then hitchhike along the auriculotemporal nerve to reach the parotid. The other divisions of the trigeminal nerve—ophthalmic (V1) and maxillary (V2)—are purely sensory and do not give rise to the auriculotemporal nerve. The vestibulocochlear nerve (CN VIII) is a separate cranial nerve altogether. Therefore, the branch carrying the parotid parasympathetics is from the mandibular division.