What is used to test for leakage at the exhaust valve of the governor?

The correct choice is to use a soap solution to test for leakage at the exhaust valve of the governor. A soap solution is effective because it allows technicians to visually identify leaks; when a gas escapes from a seal or valve, it creates bubbles in the soap solution. This method is simple, cost-effective, and provides immediate feedback, making it a valuable tool for spotting air leaks in a controlled manner.

In contrast, testing with a pressure gauge would primarily measure pressure levels, which wouldn't directly indicate whether a leak is present. A thermometer is not applicable in this situation, as it measures temperature rather than detecting air movement or leaks. Sound analysis equipment could be beneficial in detecting some types of leaks based on noise levels, but it is less direct and may not provide the clear visual evidence that a soap solution offers. Therefore, the use of a soap solution is the most suitable and efficient method for conducting this particular leakage test at the exhaust valve.