A "U" tube manometer may be filled with:

A "U" tube manometer is a device used to measure fluid pressure by comparing it between two columns of liquid. The correct choice, light oils, is suitable for use in a manometer for several reasons.

Light oils have a low density, which allows for a more manageable range of pressure measurement. They also have a relatively low viscosity, ensuring that the liquid moves smoothly and quickly responds to changes in pressure. This responsiveness is crucial for accurate readings, as it allows the manometer to equilibrate swiftly to pressure changes, providing real-time data.

Additionally, light oils do not evaporate quickly, making them stable for extended use without the risk of losing fluid through evaporation. This characteristic helps maintain consistent accuracy over time, which is essential for effective pressure measurements in refrigeration and HVAC applications.

The other options are less suitable for a manometer. Ethyl alcohol might evaporate too quickly or not provide the appropriate density for certain pressure readings. Nitrogen, being a gas, cannot be used in a liquid column manometer, as it does not create a pressure differential in the same way that a liquid does. A granular substance would not effectively form a column necessary for pressure measurement, rendering it impractical for this application. Thus, light oils are the