28. A                 measures air pressure. thermometer barometer psychrometer rain gauge

The correct answer to the question is "barometer."

A barometer is an instrument used to measure atmospheric pressure. It plays a crucial role in meteorology and helps in weather forecasting. Barometers can be classified into two main types: mercury barometers and aneroid barometers.

1. Mercury Barometer: This consists of a glass tube filled with mercury, which is inverted in a dish of mercury. The height of the mercury column changes according to the atmospheric pressure, allowing for a measurement to be taken.

2. Aneroid Barometer: This type uses a small, flexible metal box called an aneroid cell. Changes in external air pressure cause the cell to expand or contract, moving a pointer on a scale to indicate pressure values.

In contrast, a thermometer measures temperature, a psychrometer measures humidity, and a rain gauge measures the amount of rainfall. Therefore, in the context of measuring air pressure, the barometer is the appropriate tool.