Hydrometers work on [[1]] principle. 

• Hydrometer is based on Archimedes' principle, the weight of the liquid displaced by the immersed portion of the hydrometer is equal to the weight of the hydrometer.
• It is made up of a cylindrical stem with a narrow tube at the top and a spherical bulb at the bottom. Lead shots or mercury are used to partially fill the lower spherical bulb. 
• This makes it easier for the hydrometer to float or stand upright in liquids. The narrow tube is marked, allowing for easy reading of a liquid's relative density.
• The test substance is added to the glass jar. 
• The hydrometer is gently dropped into the fluid until it floats on its own. The reading against the level of liquid touching the tube gives the relative density of the liquid.

Therefore, Archimedes' principle is the correct response.