Answer: Option A.
Step-by-step explanation:
A hydrometer is an instrument used for measuring the buoyancy of water.
The hydrometer follows Archimedes' principle that says if a solid suspended in a fluid feels a buoyant force that is equal to the weight of displaced fluid by the submerged part of suspended solid.
The weight of Hydrometer sinks deeper when there is lower density of the fluid, and the stem is calibrated to calculate a numerical reading.
Thus the correct option is A.