Final answer:
The radiographic projection described is the submentovertical projection, which is utilized for examining the skull's base and the mandible by referencing the mental protuberance.
Step-by-step explanation:
The specific projection that has been performed when the patient's head and neck are hyperextended with the top of the skull against the image receptor and the central ray enters just below the chin is known as the submentovertical projection. This radiographic view is important for visualizing the base of the skull, zygomatic arches, and the mandible. The mental protuberance mentioned, which is the forward projection from the inferior margin of the anterior mandible that forms the chin, is a landmark in this type of imaging. The projection is useful in assessing the skull and cervical spine, where the articulation between the skull and spine allows for considerable extension and flexion.