What is the evacuation command during an emergency from the flight attendant?

The phrase "Brace, brace! Heads down! Stay down!" is the correct evacuation command during an emergency situation. This command is specifically designed to prepare passengers for a potential impact and provide them with clear instructions on how to protect themselves. The emphasis on "brace" indicates that passengers should position themselves to minimize injury during the emergency, while "heads down" and "stay down" provide guidance on posture that helps shield occupants from flying debris or sudden changes in cabin dynamics.

In contrast, the other options do not effectively communicate the urgency and specific actions needed during an evacuation. "Keep calm and stay seated" may promote composure but does not instruct passengers on how to prepare for impact. "Exit in an orderly fashion!" is more relevant after an emergency has occurred and the exits are open, not during the immediate crisis response. "Prepare for landing!" implies a routine action rather than an emergency situation, which would not convey the necessary urgency.