Option 1: Force - A force is a measure of the interaction between two objects. It can cause an object to accelerate or change shape. However, the bar is not a unit of force.

Option 2: Energy - Energy is the ability to do work or cause change. It is measured in joules or calories. The bar is not a unit of energy.

Option 3: Pressure - Pressure is defined as the force applied per unit area. It is commonly measured in units such as pascal (Pa) or bar. The bar is indeed a unit of pressure, and it is widely used for expressing atmospheric pressure or pressure in hydraulic systems.

Option 4: Frequency - Frequency is the number of occurrences of a repeating event per unit of time. It is measured in units such as hertz (Hz). The bar is not a unit of frequency.

In conclusion, the correct answer is option 3: pressure. The bar is a unit of pressure, commonly used in various applications.