Which of the following is a requirement for manual control of an emergency stop?

The requirement for manual control of an emergency stop is that it must involve turning off the main power supply. This is crucial because the purpose of an emergency stop is to ensure that all hazardous operations are halted immediately in case of a dangerous situation. By disconnecting the main power supply, all flame effects and related systems are quickly and effectively disabled, minimizing risk to the audience and performers.

This requirement ensures that personnel can swiftly respond to emergencies and effectively manage the safety of the environment in which flame effects are being used. Additionally, relying solely on an automated system (as suggested in one of the other options) would not provide the necessary human intervention that is often required in unexpected situations.