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During a hydrostatic test on a boiler, the safety valves must be held to their seat by means of a testing clamp to ensure that they do not open during the test. This is crucial because the purpose of a hydrostatic test is to check for leaks and the integrity of the boiler under pressure, and if the safety valve were to open, it could lead to inaccurate test results and potential safety hazards.

Using a testing clamp ensures that the safety valve is securely closed, allowing the test pressure to be applied without interference. This method is standard practice in maintaining the safety and reliability of the boiler during testing, while also ensuring that the valve's functionality can be assured when it is needed during normal operation.

The other options suggest practices that do not align with the safety requirements for testing conditions, such as setting the safety valves at their maximum allowable pressure or even adjusting them in a way that could compromise their integrity during a critical testing phase. Thus, applying a testing clamp is the appropriate and necessary action during a hydrostatic test.