ANS:C - back-sight

Foresight (FS) or Foresight reading
It is a staff reading taken on a point whose elevation has to be determined through levelling process. It is also known as minus sight. The foresight is also taken towards a change point. It is the last reading taken before the instrument is shifted. Elevation of station= height of instrument (HI) – foresight (FS) or intermediate sight (IS) Intermediate sight (IS)
It is the staff readings taken on staff held at other points whose elevations are to be determined before the foresight is taken. Intermediate sights are all staff readings between the B.S and F.S Backsight (BS) or Backsight reading
It is a staff reading taken on a point of known elevation, as on a benchmark or a change point. This is also called as a plus sight. The backsight is the first staff reading taken after the level is set up and leveled at the point.