The correct answer is: D. Focal length
The focal length is the distance between the optical center of a lens and the plane on which it projects an image of a distant object in focus. It is the distance from the front nodal point to the rear focal point.
The principal distance is the distance between the projection center and the principal plane. The principal plane is a plane perpendicular to the optical axis through the optical center of the lens.
The isocentric distance is the distance between the projection center and the isocenter. The isocenter
is a point on the optical axis such that the angle between the optical axis and a ray from the object to the isocenter is the same for all objects in the field of view.The focal length is the most important parameter of a lens, as it determines the magnification and the field of view of the image. The principal distance and the isocentric distance are less important, but they can affect the image quality.