To understand why the FRL (Filter, Regulator, Lubricator) unit is placed upstream of all control valves in a pneumatic system, let's break down its components and functions:

1. Filter: The filter removes particulates, debris, and moisture from the compressed air. This ensures that only clean air flows through the system, preventing damage and wear to downstream components.
2. Regulator: The regulator maintains the air pressure at a desired level. This prevents fluctuations in pressure that could affect the performance and efficiency of the pneumatic tools and actuators.
3. Lubricator: The lubricator adds a small amount of oil to the air. This lubrication is crucial for reducing friction and wear in the moving parts of pneumatic components, enhancing their life and operational efficiency.

The FRL unit is positioned upstream in the air supply line to ensure that all downstream components receive air that is:

• Cleaned: Particulates and moisture are removed to protect components.
• Regulated: Pressure is kept consistent for efficient operation.
• Lubricated: Proper lubrication is provided to reduce wear.

Therefore, the correct answer to the question is:

To ensure that all downstream components receive cleaned, regulated, and lubricated air.

The other options can be ruled out because:

• To supply compressed air at a constant temperature to all actuators in the system: The FRL does not control temperature.
• To provide lubrication only to the last pneumatic tool in the workshop line: The lubricator functions for all connected components, not just the last one.
• To isolate specific actuators from the main line during maintenance operations: Isolation is generally achieved through valves or other dedicated mechanisms, not the FRL unit.