Correct Answer - Option 4 : Line of collimation
Explanation:
Trunnion axis: It is the axis about which the telescope rotates in the verticle plane. It is also called the horizontal axis and transverse axis.
Optical axis: This axis is the imaginary line passing through the optical center of the object-glass and the optical center of the eyepiece.
Line of collimation: It is an imaginary line passing through the intersection of crosshairs on the diaphragm and the optical center of the objective.
