The correct answer is D. All the above.
In a truly vertical photograph, the principal point, isocenter, and plumb point coincide.
The principal point is the intersection of the optical axis of the camera and the image plane. The isocenter is the intersection of the plumb line and the image plane. The plumb line is a line that is perpendicular to the ground and passes through the camera’s optical center.
In a truly vertical photograph, the optical axis of the camera is perpendicular to the ground, so the principal point and isocenter coincide. The plumb line is also perpendicular to the ground, so the plumb point coincides with the principal point and isocenter.
Here is a diagram that illustrates the relationship between the principal point, isocenter, and plumb point in a truly vertical photograph:
[Diagram of a truly vertical photograph, with the principal point, isocenter, and plumb point labeled]
The principal point is labeled “PP”, the isocenter is labeled “I”, and the plumb point is labeled “P”.
As you can see, the principal point, isocenter, and plumb point all coincide in a truly vertical photograph.