Correct Answer - Option 1 : the change point
Explanation
Changepoint: A change point is a point selected on the route before shifting the instrument. The change point is also called a turning point. The staff is kept on the turning point and foresight is taken before shifting the instrument. The staff should not be moved when the instrument is being shifted. After the instrument has been shifted and set up at the new location, a backsight is taken on the staff still held at the change point.
(or) in simple
Change Point (CP): The point on which both the foresight and backsight are taken during the operation of levelling is called change point.
As we subtract Foresight from Height of Instrument to get RL (minus sight), We add back sight to get RL (plus sight)
Backsight (BS): The first staff reading taken by the surveyor after the levelling instrument is set up and levelled. B.S is generally taken on the point of known reduced level as on the benchmark or a change point.
Foresight (FS): The last staff reading was taken before changing the instrument to the other position. It is the staff reading taken on a point whose RL is to determined. This sight is considered as negative and deduced from Height of Instrument to determine RL of the point.
Intermediate sight: All readings taken between backsight and foresight. These are the points whose RL is determined by the method already mentioned above in FS.
