What action typically initiates door operation in an elevator system?

In an elevator system, the operation of the door is typically initiated by pressing buttons or through sensor interruptions. When a user presses a button to call the elevator or to select a floor, it sends a signal to the elevator control system, which then determines whether the elevator is at the designated floor and ready to open the doors. Similarly, if someone is detected in the doors' path by sensors, this will halt the closing process to prevent injury. This action ensures safety as well as user convenience.

The other options, while they may be parts of an elevator system's broader function, do not specifically initiate door operation. Server communication is related to the overall control system but isn't a direct action by the user. Pulling emergency levers generally serves a different purpose, such as stopping the elevator or unlocking the doors in an emergency, rather than activating door movement in normal operation. Turning off the power supply would immediately halt all operations, including door function, rather than initiate it. Therefore, pressing buttons or sensor interruptions is accurately recognized as the primary means of starting door operation in an elevator.