Correct Answer - Option 3 : the line of sight as well as the line of collimation
1.Line of sight: It is the imaginary line passing through the intersection of the crosshair on the diaphragm and the optical center of the objective lens.
when a line of sight comes in a horizontal plane, it is called a line of collimation.
The line of sight generates a vertical plane when the horizontal axis is perpendicular to the vertical axis.
2. Line of collimation: It is an imaginary line passing through the intersection of the crosshairs at the diaphragm and the optical center of the object-glass and its continuation.
3. Axis of Bubble tube: It is an imaginary line tangential to the longitudinal curve of the bubble tube at its midpoint.
4. Axis of a telescope: This axis is an imaginary passing through the optical center of the object-glass and the optical center of the eye-piece.