ETT drills are permitted only on boilers that have achieved a boiler flex level test of which level or higher?

The correct answer is C, Level 3, because ETT (Engineering Training Team) drills require that the boiler has reached a sufficient level of flexibility to ensure safe operations during drills. Level 3 represents an advanced stage of testing which indicates that the boiler has demonstrated stability and reliability under operational conditions. Achieving this level means that the system is capable of handling the stresses and potential fluctuations that can occur during drills.

Levels 1 and 2 indicate lesser degrees of testing, which may not accurately reflect the readiness of the boiler for the complexities involved in ETT drills. Level 4, while representing an advanced stage, could imply a level beyond what is required for standard drill operations. Thus, Level 3 is the appropriate threshold that balances both safety and training effectiveness for conducting ETT drills on boilers.