The correct answer is: B. Hygrometer
A hygrometer is an instrument used to measure the amount of water vapor in the air. It is a type of sensor that measures the relative humidity of air, which is the amount of water vapor in the air relative to the maximum amount of water vapor that the air can hold at a given temperature.
A hydrometer is an instrument used to measure the density of liquids. It is a type of float that measures the depth to which it sinks in a liquid. The density of a liquid is its mass per unit volume.
A lactometer is an instrument used to measure the density of milk. It is a type of float that measures the depth to which it sinks in milk. The density of milk is affected by the amount of fat and solids in the milk.
A barometer is an instrument used to measure atmospheric pressure. It is a type of gauge that measures the force exerted by the atmosphere on a surface. Atmospheric pressure is the weight of the air above a given point.