The correct answer is C. hydrometer.
A hydrometer is an instrument used to measure the specific gravity of liquids. It is a glass tube with a weighted bottom and a scale marked on the side. The hydrometer is placed in the liquid and the scale is read to determine the specific gravity.
A manometer is an instrument used to measure pressure. It consists of a U-shaped tube filled with a liquid. One end of the tube is connected to the system under pressure and the other end is open to the atmosphere. The difference in the height of the liquid in the two arms of the tube is a measure of the pressure.
A mechanical gauge is an instrument used to measure mechanical quantities such as force, torque, and pressure. It consists of a spring or other elastic element that is deformed by the quantity being measured. The deformation is then converted into a reading on a scale.
A psychrometer is an instrument used to measure the humidity of air. It consists of two thermometers, one of which has a wetted wick around the bulb. The difference in the readings of the two thermometers is a measure of the humidity.