The correct answer is: C. Perspective centre
The principal line is the line joining the principal point and the perspective centre. The principal point is the intersection of the optical axis and the image plane. The perspective centre is the point at which the optical axis intersects the ground plane.
The nadir is the point on the ground directly below the camera. The isocenter is the point on the image plane that is equidistant from the principal point and the perspective centre.
The principal line is important because it is used to calculate the focal length of the camera. The focal length is the distance between the principal point and the optical centre of the lens. The focal length determines the angle of view of the camera.
The principal line is also used to calculate the perspective distortion of the camera. Perspective distortion is the effect that makes objects that are further away from the camera appear smaller than objects that are closer to the camera.
The principal line is a useful tool for understanding how cameras work and for correcting perspective distortion.