Correct option is A
Levelling is the process by which elevations of points or differences in elevation are determined.
Types of levelling instruments used in surveying are:
Auto-level:
An automatic level is an optical instrument used to establish or verify points in the same horizontal plane in a process known as levelling.
Its provides consistent level accuracy more quickly than many other leveling instruments.
Auto level has internal compensator mechanism to automatically adjust the line of sight.
Tilting level:
A surveying instrument with sighting telescope so mounted that it can be raised or lowered through a limited arc without impairing accuracy of reading, though axis of rotation is not precisely horizontal.
Its function is to determine the distance between the point where the line of collimation (line-of-sight) of the level intersects the staff and the next lower staff graduation, and thus eliminates the need for estimation.
Abney level:
An Abney level is an instrument used in surveying which consists of a fixed sighting tube, a movable spirit level that is connected to a pointing arm, and a protractor scale.
An internal mirror allows the user to see the bubble in the level while sighting a distant target.
They are used to measure degrees, percent of grade and topographic elevation.
Wye-level:
An instrument, consisting of a spirit level mounted under and parallel to a telescope, that can be rotated in its Y-shaped supports for adjustment.
The wye level has an advantage over the dumpy level in the fact that the adjustments can be tested with greater rapidity and ease.
