What indicates that passengers should brace for impact?

The correct answer highlights that a specific announcement from the captain or lead flight attendant serves as a direct and clear instruction for passengers to brace for impact. This communication is vital because it ensures all passengers receive the same message at once, reducing confusion and enhancing the likelihood of an effective response. The captain or lead flight attendant is responsible for delivering critical safety information, and their authority lends weight to the instruction.

While other options might suggest situations that could indicate an emergency, they do not provide the same level of clarity or authoritative direction. For instance, a sudden cabin pressure change may signal a need to prepare, but it lacks the explicit instruction for passengers to brace. A loud alarm sound might indicate danger, yet without context, it does not automatically convey the need to adopt a bracing position. Lastly, while seeing flight attendants by their exits can imply an emergency, it doesn't convey a clear instruction on what passengers should do next. The specific announcement is essential for ensuring that everyone is aware and ready when necessary.