A __________ solution can be used around the threads of a fitting to check for leakage.

Using a soap solution around the threads of a fitting is an effective method to check for leaks in pneumatic systems. When soap is mixed with water, it creates bubbles when air escapes through a leak. This visual indication allows technicians to easily identify the exact location of the leak. The suds produced by the soap offer a clear and immediate sign of escaping air, making it a reliable and widely used technique in pneumatic troubleshooting.

While other substances are available, they do not serve the same purpose as a soap solution. Detergents can serve similar functions but are not specifically designed for leak detection and may vary in effectiveness. Alcohol and lubricants, on the other hand, do not effectively produce visible bubbles when air escapes, making them unsuitable for checking for leaks in fittings. This specificity of soap solutions for leak detection is why it is considered the correct answer.