The correct answer is: C. Principal point
The principal point is the intersection of the optical axis of the camera and the image plane. It is the center of the perspective projection of the camera’s optical center onto the image plane. The scale of a vertical photograph is the ratio of the distance between two points on the ground to the distance between their images on the photograph. The scale of a tiled photograph is the same as the scale of a vertical photograph along the photo parallel through the principal point.
The isocenter is the point on the ground where the plumb line from the camera’s principal point intersects the ground. The plumb point is the point on the photograph where the plumb line from the camera’s principal point intersects the photograph.
The principal point is the only point on the photograph that is common to all photo parallels. Therefore, the scale of a vertical photograph is the same as the scale of a tiled photograph along the photo parallel through the principal point.