Which component primarily allows the valve plug to move in a balance valve?

The primary factor that allows the valve plug to move in a balance valve is pressure differences. In a balance valve, the design typically utilizes a mechanism where the flow of fluid creates varying pressures on different sides of the valve plug. This pressure differential acts to position the plug appropriately, either opening or closing the valve to regulate the flow. The balance between these pressures is critical for the valve's operation, enabling it to adjust the flow based on the conditions of the system effectively.

While thermal expansion might affect components in terms of thermal loads, it does not directly cause movement in the valve plug of a balance valve. Pneumatic force could influence some types of valves but is not the operating principle within a balance valve context. Similarly, magnetic pull pertains to valves that are specifically designed for actuation using magnetic forces, which is not the principle underlying the functionality of balance valves. Thus, understanding the critical role of pressure differences clarifies why this is the correct answer.