A pneumatic control system is primarily characterized by:

A pneumatic control system is fundamentally based on the use of compressed air signals to perform control functions. This system operates by using the pressurized air to transmit signals and control actions, such as opening or closing valves and moving actuators. The reliance on air pressure makes pneumatic systems distinct from other types of control systems, such as electric ones, which utilize electrical signals instead.

The correct answer emphasizes the nature of pneumatic systems, where the flow and pressure of compressed air are manipulated to achieve desired results. The effectiveness and speed of pneumatic control systems can be attributed to their ability to quickly transmit signals and actuate machinery using compressed air, making them suitable for various applications such as automation in manufacturing and other industries.

In contrast to other options, electric signals pertain to electrical control systems, while fluid mechanics, although a relevant principle, encompasses broader concepts beyond just pneumatic systems. Mechanical parts are utilized in many types of systems, but they do not define the characteristic functionality of pneumatics, which is primarily about the control via compressed air. Therefore, understanding that pneumatic systems capitalize on compressed air to operate is key to distinguishing them within the broader context of control systems.