To answer this question, the user needs to know what humidity is and how it is measured. Humidity refers to the amount of moisture or water vapor in the air. It is measured using an instrument called a hygrometer.
Option A, Barometer, is not the correct answer because a barometer is used to measure atmospheric pressure, not humidity.
Option B, Thermometer, is not the correct answer because a thermometer is used to measure temperature, not humidity.
Option C, Hygrometer, is the correct answer because a hygrometer is specifically designed to measure humidity. It works by measuring the amount of moisture in the air and expressing it as a percentage of the maximum amount of moisture that the air can hold at a given temperature.
Option D, Hydrometer, is not the correct answer because a hydrometer is used to measure the density of a liquid or the specific gravity of a substance, not humidity.
Therefore, the answer is: C. Hygrometer.