Which of the following instruments is used for measuring the relative density of liquids?

Option 1 -> Lactometer is a specialized hydrometer designed specifically to measure the density of milk to check its purity.

Option 2 -> Decimeter is a unit of length measurement (10 centimeters), not an instrument.

Option 3 -> Hydrometer is an instrument specifically designed to measure the relative density (specific gravity) of liquids by flotation principle.

Option 4 -> Orthometer is a geodetic instrument used for measuring orthometric heights/elevations, not liquid density.

Hence, Option 3: Hydrometer -> A hydrometer works on Archimedes' principle and floats at different levels in liquids of different densities, with calibrated markings indicating the relative density directly -> correct