The correct answer is D. cross staff.
A line ranger is a device used to measure distances. It consists of a telescope and a graduated rod. The telescope is used to sight the rod, and the graduated rod is used to measure the distance between the telescope and the rod.
A steel tape is a flexible measuring tape made of steel. It is used to measure distances up to 100 feet.
An optical square is a tool used to measure perpendicular offsets. It consists of a telescope and a spirit level. The telescope is used to sight the object to be measured, and the spirit level is used to ensure that the telescope is perpendicular to the ground.
A cross staff is a tool used to measure angles. It consists of two arms that are perpendicular to each other. The arms are marked with degrees, and the angle between the arms can be measured by sighting the two arms on the object to be measured.
Of the four options, only the cross staff is not used in measuring perpendicular offsets.