Air supplied to a pneumatic system is supplied through the __________ line.

The supply line is the correct answer because it refers specifically to the pathway through which pressurized air is delivered to the pneumatic system. This line is crucial for the operation of pneumatic tools and machinery, providing the necessary force and movement. By using the supply line, the system can effectively harness the power of compressed air to perform various tasks, such as moving actuators or powering tools.

In contrast, the intake line typically refers to the entry point for air into a compressor, while the actuator line is associated with directing air specifically to an actuator which converts the air's energy into mechanical motion. The exhaust port, on the other hand, allows the release of air from the system after it has done its work, emphasizing its function as an outlet rather than a supply point. This distinction clarifies the pivotal role the supply line plays in the functionality of pneumatic systems.