When a broker calls off at the very instant a dealer hits the broker's price:

Under the ACI Model Code governing voice broking and dealing ethics, a broker's price quote remains firm and executable only until it is withdrawn or "called off" by the broker. If a broker calls off a rate at the exact instant a dealer attempts to hit or take that price, the quote is considered effectively withdrawn prior to acceptance. In financial market practice, mutual consent and clear acceptance of a valid offer are necessary to create a binding contract. Because the offer was terminated simultaneously with the acceptance attempt, no valid transaction exists, and the deal should not be concluded. Dealers and brokers must respect this rule to ensure orderly and fair market conduct.

Option B: The broker does not have discretionary authority to decide whether a deal stands when a call-off occurs simultaneously with a hit.

Option C: ACI's Committee for Professionalism provides dispute resolution guidance and code enforcement, but does not adjudicate individual real-time trade conclusions.

Option D: Concluding the transaction would violate market rules, as an offer that is called off simultaneously cannot result in a legally binding trade.

- ACI Financial Markets Association, "ACI Model Code: The International Code of Conduct and Practice for Financial Markets", Section: Brokers / Calling off prices and deal closure, p. 45-48.

- ACI Financial Markets Association, "ACI Dealing Certificate Study Guide", Topic: Inter-dealer broking standards and trade execution, 2020.