Final answer:
The primary landmark used in Eisenstein's Method for Sagittal Canal Measurement is the foramen magnum (A), a large opening in the occipital bone of the skull.
Step-by-step explanation:
The primary landmark used in Eisenstein's Method for Sagittal Canal Measurement is the foramen magnum, which is answer A). The foramen magnum is a large opening in the occipital bone of the skull through which the spinal cord emerges, and the vertebral arteries enter the cranium. It is crucial in anatomical studies and clinical applications related to the spinal cord and brainstem.
The atlas vertebra (C1), while significant as it supports the skull, is not the primary landmark for the method mentioned. Similarly, the coccyx and the sacrum play important roles in the vertebral column but are not the primary landmarks for Eisenstein's Method.