ANS:A - line of sight

Concept:

Line of sight: The sighting or pointing line of a telescope, defined by the optical centre of the objective and the intersection of crosshairs.

Line of collimation: It is an imaginary line passing through the optical centre of the objective glass and its continuation.

Axis of Telescope: The axis is an imaginary line passing through the optical centre of the object-glass and the optical centre of the eye-peace.

Axis of the Bubble Tube: It is an imaginary line tangential to the longitudinal curve of the bubble tube at its middle point.

If we talk about a single point on Earth when the bubble is central, then a line of sight, line of collimation and axis of telescope lies on same plane and line, but in a large survey or geodetic surveys, with change or shift of level from station to station, line of collimation differs, and won't b horizontal to the previous station, so we say a line of sight should b parallel when Buble is at the middle, for which it's not important whether horizontal as per previous stations.

So, option A is correct answer. Line of sight and line of collimation is different. Line of sight depends on the purpose of survey but the line of collimation is always horizontal. Purpose of the survey mean when you find the height of anything then there you may clear about these two things. In levelling operation, the first sight on any change point is back sight. The second sight on any change point is an intermediate sight. The line commences with an intermediate sight and closes a fire sight.