Final Answer:
The fluid used in the manometer is typically mercury (3).
Thus the correct option is 3 mercury.
Step-by-step explanation:
Manometers are devices used to measure pressure, and the choice of fluid depends on the application and pressure range.
Mercury is often preferred for its high density, which allows for precise pressure measurements in various systems.
It remains liquid at room temperature and is not easily compressible, providing accurate and stable readings.
Mercury manometers are commonly employed in situations where high precision is essential, such as laboratory experiments and industrial processes.
The vertical displacement of mercury in a U-shaped tube corresponds directly to the pressure difference between the two ends of the manometer.
While mercury is a common choice, other fluids like water or oil may be used in specific cases where the pressure range is lower, or where the use of mercury is restricted due to environmental concerns.
Gasoline is not typically used in manometers as it is flammable and poses safety risks.
The selection of the fluid depends on factors like density, volatility, and safety considerations, ensuring that the manometer is suitable for the intended purpose and environment.
Thus the correct option is 3 mercury.