The U.S. rule that halted trading after the 1987 crash was Rule 80B.
Rule 80B was adopted by the U.S. Securities and Exchange Commission after Black Monday to establish procedures for extraordinary market declines. It permitted coordinated trading halts when specified market thresholds were reached, giving investors and exchanges time to process information instead of allowing automated selling to continue without interruption.
The rule reflected lessons from October 19, 1987, when the Dow fell 22.6 percent and market infrastructure came under severe pressure. Later versions of the system evolved into the modern circuit-breaker framework used in U.S. equities.
Rule 10b-5 is a common wrong answer because it is also an important SEC rule, but it concerns securities fraud and deceptive practices. Rule 80B was specifically associated with coordinated emergency halts and the post-1987 response to market-wide crashes.