Upon first indication of an unusual situation, what command should be shouted?

The command "Brace, brace, brace" is crucial during an unusual situation, particularly in cases where an impact or emergency landing is imminent. This command alerts passengers to assume a protective position to minimize injury. It conveys urgency and prepares everyone on board for the potential risk associated with the situation at hand, clearly indicating that they need to brace themselves for impact.

The other commands, while important in various contexts, serve different purposes. "Assist, assist, assist" would generally be used in a situation where additional help is required, possibly for managing injuries or aiding passengers. "Secure, secure, secure" may be relevant in situations needing immediate stabilization of items or equipment but does not specifically alert passengers to a brace position. "Prepare, prepare, prepare" suggests readiness but lacks the specificity required for actions during imminent danger. Therefore, "Brace, brace, brace" is the most appropriate choice in this scenario as it directly addresses the action passengers need to take for their safety.