The correct answer is: C. Nadir point
The nadir point is the point on the ground directly below the camera when the camera is pointed straight down. It is the point where the vertical line passing through the perspective center intersects the plane of the photograph.
The photo plumb point is the point on the photograph where the plumb line from the perspective center intersects the photograph. The plumb line is an imaginary line that passes through the perspective center and is perpendicular to the ground.
The isocenter is the point on the photograph where the two principal axes intersect. The principal axes are the two lines that pass through the perspective center and are perpendicular to each other.
The nadir point is the most important point in a photograph because it is the point that defines the orientation of the photograph. The other points, such as the photo plumb point and the isocenter, are derived from the nadir point.