ANS:D - 100 m

The axis of the bubble tube is not an axis passing through the bubble tube-like telescope. It's an arc that passes through the midpoint of the bubble tube length. For a deflection of theta degree at the center of the earth what deflection it makes on the surface of the earth is called its sensitivity. Similarly, the arc of the bubble tube also has a center at the center of the earth and so the radius of curvature is generally taken as 100m. Greater is the radius of the Curvature lesser is the sensitivity.

Sensitivity = Length of one division on tube / Radius of curvature.
= L/R.

If R is more, sensitivity is less. In the survey, it is important to have greater sensitivity for greater accuracy. Sensitivity is defined by the level tube, it changes with many factors like the viscosity of fluid, radius of curvature of the bubble, dia of the tube, the internal roughness of tube etc. Greater is the radius of curvature greater is the sensitivity. The radius of curvature of the arc bubble is very sensitive when if you put it the center in experiment. It will give you correct image of line of sight. Bubble tube is generally to fix at 100m distance.