Which of the following is used to measure water levels accurately?

The air line water level measuring device is specifically designed to measure water levels accurately. This device operates by using a small tube that is open at one end and submerged in the water while the other end is connected to a gauge or measurement tool at the surface. As the water level changes, the pressure of the air in the tube changes accordingly, which can be easily measured to give an accurate reading of the water level.

This form of measurement is particularly valuable because it can provide precise data, is relatively easy to install, and minimizes disturbances in the water body being measured. In contrast, the other options either do not serve the purpose of measuring water levels directly or are not designed for accuracy in this specific context. For instance, a gravel tube is typically used for settling solids rather than measuring water levels, while suction lines are used to draw water rather than measure its height. The pump motor base seal does not pertain to water level measurement at all, as it is not a measuring device.