Which device is used for the API gravity fuel oil test?

The correct choice for the device used for the API gravity fuel oil test is the hydrometer. This instrument is specifically designed to measure the relative density or specific gravity of liquids, which is essential for determining the API (American Petroleum Institute) gravity of petroleum products like fuel oil.

API gravity is a measure that indicates how heavy or light a petroleum liquid is compared to water. A hydrometer works by floating in the liquid, with its scale providing readings based on how deep it submerges. This reading allows for the calculation of specific gravity, which can be converted to API gravity.

While a thermometer measures temperature, which is not directly related to the density measurement, and a manometer is used to measure the pressure of gases or liquids, they are not suitable for evaluating the API gravity. Similarly, a graphometer is a surveying instrument used for measuring angles; it does not have the capability to provide measurements related to fluid density. Thus, the hydrometer is the preferred and proper tool for this specific test.