The correct answer is option 3: Pressure.
In physics, the unit of pressure is called a bar. It is a commonly used metric unit for measuring the amount of force applied per unit area. Pressure is defined as the force exerted on a surface per unit area. It is measured in units of force divided by units of area.
Option 1: Force - A bar is not a unit of force. Force is measured in units such as newtons or pounds.
Option 2: Energy - A bar is not a unit of energy. Energy is measured in units such as joules or calories.
Option 4: Frequency - A bar is not a unit of frequency. Frequency is measured in units such as hertz or cycles per second.
Therefore, the correct answer is option 3: Pressure. A bar is a unit of pressure.