The correct answer is C) Secure and tag the supply circuit breaker in the open position. This is the correct answer because it is the proper safety precaution to take before working on any electrical or electronic equipment. Securing and tagging the supply circuit breaker in the open position ensures that the circuit is de-energized and prevents accidental re-energization, which could lead to serious injury or damage. The other options are incorrect because A) stationing a man at the circuit supply switch does not physically disconnect the power, B) bypassing interlocks defeats important safety mechanisms, and D) de-energizing the applicable switchboard bus may not isolate the specific circuit where work is being performed.