The correct answer is D. all of the above.
The horizontal cross hair is a thin line that is used to sight on objects. It is located in the center of the telescope and is parallel to the optical axis of the instrument. The horizontal cross hair must be adjusted so that it is perfectly horizontal in order to obtain accurate measurements.
When the instrument is used for leveling, the horizontal cross hair must be adjusted so that it is parallel to the ground. This is done by using a spirit level to ensure that the instrument is level.
When the instrument is used for prolonging a straight line, the horizontal cross hair must be adjusted so that it is parallel to the line being prolonged. This is done by sighting on two points on the line and adjusting the horizontal cross hair until it is centered between the two points.
When the instrument is used for measurement of horizontal angles, the horizontal cross hair must be adjusted so that it is parallel to the line of sight. This is done by sighting on two points and adjusting the horizontal cross hair until it is centered between the two points.
If the horizontal cross hair is not adjusted properly, it will cause errors in the measurements. This is because the horizontal cross hair is used to sight on objects, and if it is not parallel to the ground or the line being prolonged, it will not be in the correct position to sight on the object. This will cause the measurements to be inaccurate.