Concept:The optical square is a surveying instrument designed to set out lines at right angles to a main chain line.
Explanation:In chain surveying, details on the ground are located by measuring perpendicular offsets from the chain line.
The optical square helps the surveyor establish an exact
90∘ direction from the chain line to the object.
It does this using the principle of double reflection, either through a mirror method or a prism square method.
This makes it the correct instrument for setting out right angles in the field.
The other options are unsuitable because station pegs mark survey stations, offset staff is used for measuring short offsets, and a steel band is used for linear measurement.
Answer:A. Optical square