A barometer is used to measure which one of the following?

atmospheric pressure

The correct answer is option 2. atmospheric pressure.

A barometer is an instrument used to measure atmospheric pressure, which is the force exerted by the weight of air in the atmosphere. It consists of a sealed glass tube partially filled with mercury or another liquid, with the open end submerged in a reservoir of the same liquid. Changes in atmospheric pressure cause the level of the liquid in the tube to rise or fall, providing a measure of the pressure. Atmospheric pressure is an important parameter in weather forecasting and is also used in various scientific and engineering applications.