Lactometer is a device use to test the purity of milk by the virtue of specific gravity of cow's milk according to standards.

Lactometer is graduated into hundred parts.

Lactometer is an instrument used for measuring the purity of milk.

As lactometer sinks less in case of B , we can easily say that the density of sample B is more than that of sample A , hence water content in A would be more as milk is denser than water .

Hydrometer is an instrument used for determining the density of a liquid. It usually consists of a glass float with a long thin stem which is graduated. The glass float is a large hollow bulb which increases the buoyancy so that the hydrometer floats. The narrow stem increases the sensitivity of the hydrometer. The bottom of the hydrometer is made heavier by loading it with lead shots so that it floats vertically.

A hydrometer is an instrument used to measure the relative density (or specific gravity) of liquids that is, the ratio of the density of the liquid to the density of water.

A hydrometer is used to test the state of charge of a battery cell. This is performed by measuring the density of the electrolyte, which is accomplished by measuring the specific gravity of the electrolyte. The greater the concentration of sulfuric acid, the more dense the electrolyte becomes.

The narrow stem in a hydrometer increases its sensitivity.
The pressure increase with depth, so the lower part is made heavy to withstand high pressure.

The lower end of a hydrometer is filled with mercury or lead in order to make it heavy so that it remains upright 

Hydrometer is a device used to measure the specific gravity or density of a substance as an application of Archimedes principle.

Hydrometer is a device used for directly measuring the relative density of a liquid with respect to water and hence helps to test the liquids purity. A hydrometer tube submerges less in a liquid of higher relative density and more in a liquid of lower relative density with regard to water. Thus the graduations on the hydrometer test tube show increase in relative density in the downward direction as it sinks less in a liquid of higher relative density.

Hydrometer is based on the principle of floatation i.e., the weight of the liquid object displaced by the immersed portion of the hydrometer is equal to the weight of the hydrometer. 

Common hydrometer consists of three main parts the stem, the large bulb (floating bulb) and the small bulb filled with mercury or lead shots (gravity bulb). The floating bulb increases the upthrust on the hydrometer because when it submerges in the water completely it increases the weight of the water displaced. The smaller bulb is filled with lead shots or mercury which helps in lowering the center of gravity which enables the hydrometer to float vertically in the liquid. Thus the smaller bulb is called the gravity bulb. The narrow stem helps the hydrometer to sink effectively when dropped in a liquid with higher or lower relative density thus increasing the sensitivity of the hydrometer.

Hydrometer is a devide used to determine the specific density of liquids.

 archimedes principle states that weight lost by the body partially or fully immersed in a liquid is equal to the weight of the liquid displaced and hydrometer works on this principle.

Hydrometer is an instrument used for measuring the density of liquids.

Answer is A