Question

_____ is an instrument used for measuring the density of a liquid density.
A) Lactometer
B) Hydrometer
C) Barometer
D) None

Answer 1

Device which is used to measure the liquid density works on principle of floatation, and is also used to test the purity of the water. The other name of the device is aerometer.

Complete step by step answer:
A hydrometer or aerometer is a device that measures the specific gravity (relative density) of a fluid - the ratio of the density of the liquid to the density of water. A hydrometer is usually made of glass, and consists of a cylindrical stem and a bulb that is weighed with mercury or lead shot so that it can float upright.

Hence option B is correct

Additional info : A hydrometer is a device used to measure the relative density of liquids based on the idea of ​​buoyancy. They are usually calibrated and end in one or more scales of specific gravity. A hydrometer is usually made of glass, consisting of a cylindrical rod, a bulb that is weighed with mercury or a lead frame, to float it upright.
Hydrometer as a device works on the principle of Archimedes. Meters (seen as vertical shafts) based on the density of the fluid will sink more or less. Generally, the shaft is calibrated to give readings directly. However, hydrometers are catering for specific ranges of fluid densities, depending on the density of the fluids.

Notes : The hydrometer does not give us direct density. It measures the specific gravity (ie, the density of water relative to water). We need to multiply the density of the water by the reading obtained from this instrument to get the absolute density of the liquid.