The correct answer is: A. equidistant from A and B.
The difference in elevation between two points can be determined using a variety of methods, including trigonometry, leveling, and barometric leveling. Leveling is the most common method, and it involves using a level to measure the difference in elevation between two points. The level is placed on a tripod at a point that is equidistant from the two points, and the difference in elevation is then determined by reading the level at each point.
If the level is not placed equidistant from the two points, the difference in elevation will be inaccurate. This is because the level will be measuring the difference in elevation between two points that are not at the same height, and this will introduce an error into the measurement.
Therefore, the instrument station should be equidistant from A and B in order to obtain the most accurate results.
Option B is incorrect because the instrument station should be equidistant from A and B, not closer to the higher station. If the instrument station is closer to the higher station, the difference in elevation will be underestimated.
Option C is incorrect because the instrument station should be equidistant from A and B, not closer to the lower station. If the instrument station is closer to the lower station, the difference in elevation will be overestimated.
Option D is incorrect because the instrument station should be equidistant from A and B, not as far as possible from the line AB. If the instrument station is as far as possible from the line AB, the difference in elevation will be inaccurate.