The correct answer is: A. Base line of the left photograph of stereo pair.
The base line is the distance between the two camera centers in a stereo pair. The minor control point is a point on the object that is imaged on both photographs in the stereo pair. The principal point is the center of projection of the camera. The distance between the minor control point and the principal point is equal to the base line of the left photograph of the stereo pair.
Option B is incorrect because the base line of the right photograph of the stereo pair is not necessarily equal to the base line of the left photograph. The base line can be different for each photograph in a stereo pair.
Option C is incorrect because the sum of the base lines of the stereo pair is not necessarily equal to the distance between the minor control point and the principal point. The sum of the base lines is equal to the distance between the two camera centers, which is not necessarily equal to the distance between the minor control point and the principal point.
Option D is incorrect because the mean of the base lines of the stereo pair is not necessarily equal to the distance between the minor control point and the principal point. The mean of the base lines is equal to the average of the distances between the two camera centers, which is not necessarily equal to the distance between the minor control point and the principal point.