Concept:Relative density of a liquid is conveniently determined by a hydrometer.
Explanation:A hydrometer is a calibrated float that sinks to a certain depth in a liquid.
The depth of sinking depends on the relative density of the liquid.
When placed in an acid, the hydrometer reading directly gives the relative density of the acid.
A manometer is designed to measure pressure differences in fluids.
A hypsometer is used to determine the boiling point of a liquid or to measure altitude.
A hygrometer measures the moisture content or humidity of the air.
These other instruments cannot give the relative density of an acid.
Therefore, the relative density of an acid is measured using a hydrometer.
Answer:B. hydrometer