What signals that passengers should brace for landing?

A specific brace signal is the correct indication that passengers should brace for landing because it provides a clear and universally understood directive during a critical phase of flight. This signal is typically communicated through the flight crew using specific techniques such as verbal commands, hand signals, or tones that clearly convey the need for passengers to adopt a bracing position to reduce the risk of injury during an emergency landing.

While other options may serve as cues or instructions in various situations, they do not specifically denote the immediate need for passengers to brace. Dimming cabin lights might prepare passengers for landing but does not directly communicate the need to brace. A voice announcement could indicate brace positions, but it needs to be a direct instruction. The activation of the seatbelt sign indicates that passengers should fasten their seatbelts but does not explicitly convey the need for a brace position. Therefore, the specific brace signal is the key indicator in such situations.