Atmospheric humidity is measured by

The correct answer is C. Hygrometer.

A hygrometer is an instrument used to measure the amount of water vapor in the air. 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 dry bulb and one with a wet bulb. The wet bulb is covered with a wet cloth, and the evaporation of water from the cloth cools the wet bulb thermometer. The difference in temperature between the wet and dry bulb thermometers is used to calculate the relative humidity.

An auxanometer is an instrument used to measure the growth of plants. A photometer is an instrument used to measure the intensity of light.

I hope this helps!