The correct answer is: A. Focal length of the camera
Parallax is the apparent displacement of an object relative to a background when viewed from different positions. In the context of aerial photography, parallax is used to measure the difference in elevation between two points. The greater the difference in elevation, the greater the parallax.
The focal length of a camera is the distance between the lens and the image sensor. The focal length determines the angle of view of the camera. A longer focal length will result in a narrower angle of view, while a shorter focal length will result in a wider angle of view.
The overall size of a photograph does not affect the parallax. The percentage of overlap between two photographs does affect the parallax, but only to a small degree.
Therefore, the only option that is independent of the parallax is the focal length of the camera.