If the plane table is not horizontal in a direction at right angles to the alidade, the line of sight is parallel to the fiducial edge only for

ANS:A - horizontal sights

An alidade is a device that allows one to sight a distinct object and use the line of sight to perform a task. This task can be, for example to draw a line on a plane table in the direction of the object or to measure the angle to the object from some reference point.

The line of sight is parallel to the fiducial edge only for horizontal angle because plane table inclined perpendicular to line of sight.