Final answer:
The mental foramen is the structure that would be projected closest to the occlusal plane on a mandibular posterior periapical radiograph due to its location on the anterior-lateral side of the mandibular body.
Step-by-step explanation:
Of the structures listed, the mental foramen would be projected closest to the occlusal plane when taking a mandibular posterior periapical radiograph. The mental foramen is an opening located on the anterior-lateral side of the mandibular body which transmits the mental nerves and vessels. Being located on the body of the mandible and given its relative position, it is more likely to be projected closer to the occlusal plane compared to the other structures listed which are located either higher up or further back in relation to the mandible. The external oblique ridge and the mandibular canal are located more superiorly or posteriorly and the submandibular salivary gland fossa is located inferiorly, making them less likely to be projected closest to the occlusal plane in the specified radiograph.