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 very important tool for meteorologists, as it helps them to predict the weather. Hygrometers are also used in many other industries, such as agriculture and manufacturing.
There are many different types of hygrometers, but the most common type is the wet-and-dry bulb hygrometer. This type of hygrometer consists of two thermometers, one with a wet bulb and one with a dry bulb. The wet bulb is covered with a wet cloth, and the dry bulb is not. The difference in the temperature of the two bulbs is used to calculate the relative humidity.
Other types of hygrometers include the hair hygrometer, the dew point hygrometer, and the psychrometer.
A hydrometer is an instrument used to measure the density of liquids. It is a very important tool for chemists, as it helps them to determine the concentration of solutions. Hydrometers are also used in many other industries, such as brewing and winemaking.
A barometer is an instrument used to measure atmospheric pressure. It is a very important tool for meteorologists, as it helps them to predict the weather. Barometers are also used in many other industries, such as aviation and construction.
A mercury thermometer is an instrument used to measure temperature. It is a very common type of thermometer, and it is used in many different industries. Mercury thermometers are not as common as they used to be, as mercury is a toxic substance.