I would say the answer is D
A hygrometer is an instrument used to measure relative humidity. ... To determine the relative humidity, the temperatures are taken from the dry-bulb thermometer and the temperature difference between the wet and dry bulb thermometers.
hygrometer is a weather instrument used to measure the amount of humidity in the atmosphere. There are two main types of hygrometers -- a dry and wet bulb psychrometer and a mechanical hygrometer.