Which of the following is a method to create a hydrometer?

A hydrometer is an instrument used to measure the specific gravity or density of a liquid relative to water. The correct method to create a hydrometer involves using a tall clear container to hold the calibration solution, as this allows for the accurate and consistent measurement of the liquid's density. The tall design helps minimize the effects of surface tension, allowing the hydrometer to float freely and provide a reliable reading.

In practice, once the hydrometer is calibrated in this container with a known solution, it can then be used in various liquids to determine their density based on how deep the hydrometer sinks. This setup also allows observers to clearly see the level at which the liquid intersects the scale on the hydrometer, further ensuring measurement accuracy.

While mixing sugar with water can create a solution with varying densities, it is not a method for constructing a hydrometer itself. Removing air from the calibration container does not directly create the hydrometer and may introduce other complications in measurement. Heating the saltwater solution could change its density and affect the results, but it is not a necessary step for creating the hydrometer.