One of the following surveying instruments is used to set out lines at right angles to the main chain line

The surveying instrument used to set out lines at right angles to the main chain line is an optical square. An optical square is a simple, yet precise surveying instrument that consists of two mirrors that are set at exactly 90 degrees to each other. It is used to establish perpendicular lines or right angles to the main chain line during the surveying process. The instrument is set up by placing it on the ground with one mirror facing down the main chain line and the other mirror facing at right angles to the main chain line. To use the optical square, a surveyor would first set up the main chain line using a steel band or another measuring instrument. Once the main chain line has been established, the optical square is set up and aligned with the main chain line. The mirror facing down the main chain line reflects the view of the surveyor along the line, while the other mirror reflects the view at right angles to the main chain line. This allows the surveyor to set out lines at right angles to the main chain line with a high degree of accuracy. In summary, the optical square is a surveying instrument used to set out lines at right angles to the main chain line by reflecting views along the main chain line and at right angles to it.