Which instrument is used to measure atmospheric pressure?

The correct option is Barometer.

Explanation and Step-by-Step Breakdown:

1. Understanding Atmospheric Pressure:
Atmospheric pressure is the force exerted by the weight of the air above us per unit area of the Earth's surface. Measuring this pressure is essential in meteorology for forecasting weather changes, as changes in atmospheric pressure usually indicate shifting weather patterns.

2. Identifying the Correct Instrument:
A barometer is a scientific instrument specifically designed and used to measure atmospheric pressure. Mercury barometers and aneroid barometers are the most common types used for this purpose.

3. Reviewing the Other Instruments:
• Hygrometer: Used to measure the humidity or moisture content in the atmosphere.
• Ammeter: Used to measure electric current in a circuit.
• Altimeter: Used to measure altitude or height above sea level (though often based on pressure, its primary function is height measurement).

Therefore, the instrument used to measure atmospheric pressure is the Barometer.