Concept:The instrument used to lay out perpendicular offsets from a chain line is the optical square.
Explanation:In chain surveying, the main chain line is first established.
To locate details such as field boundaries, surveyors need to set out lines at right angles to this chain line.
The optical square works by using mirrors or a prism to give an exact
90∘ angle.
Thus, the surveyor can fix points and measure perpendicular offsets accurately.
Station pegs only mark positions, offset staff measures short offsets, and steel band measures distance.
Therefore, the correct instrument for setting out right angles is the optical square.
Answer:A. Optical square