Homologous point is A. Photo principal point B. Ground principal point C. Ground isocenter D. All the above

The correct answer is: A. Photo principal point.

A homologous point is a point on the ground that corresponds to a point on the photograph. The photo principal point is the intersection of the optical axis of the camera and the image plane. The ground principal point is the intersection of the central ray of the camera and the ground. The ground isocenter is the intersection of the vertical axis of the camera and the ground.

The photo principal point is the only point on the photograph that is guaranteed to correspond to a point on the ground. The ground principal point and the ground isocenter are only guaranteed to correspond to points on the ground if the camera is perfectly aligned with the vertical and horizontal axes.

In practice, the photo principal point is usually used to represent the homologous point. This is because the photo principal point is the only point on the photograph that is easy to measure. The ground principal point and the ground isocenter are more difficult to measure, and they are not always guaranteed to correspond to points on the ground.