The instrument used for measuring air pressure is called:
Barometer.
Explanation for correct option:
Option B:
Explanation for incorrect options:
Option A:
Anemometer is a device that is used to calculate the speed of wind.
Option C:
A hygrometer is a specially designed device that is used to calculate the relative humidity in an open or enclosed area.
Option D:
A thermometer is a device that is used for measuring the temperature.
Thus, the correct answer is option B.