The correct answer is: A. Parallax.
Parallax is the apparent displacement of an object relative to a background when viewed from different positions. It is caused by the difference in the angle of view from each position.
A parallax bar is a tool used to measure parallax. It consists of two rods, one of which is fixed and the other of which is movable. The movable rod is placed in front of the object to be measured, and the angle between the two rods is measured. The parallax angle is then used to calculate the distance to the object.
The other options are incorrect because they do not accurately describe what a parallax bar measures. Height is a physical quantity that is measured in units such as meters or feet. Parallax difference is the difference between the parallax angles measured from two different positions. Height difference is the difference between the heights of two objects.