When an aircrew receives the command 'Knock it off,' what action should they take?

The command 'Knock it off' is a critical directive used in aviation contexts, particularly during training exercises or combat scenarios. When this command is issued, it signifies that aircrew members must cease all air combat maneuvers immediately. This command is designed to ensure safety and to halt any actions that could lead to potential accidents or unsafe situations among aircraft involved in training or in operational environments.

Ceasing all air combat maneuvers allows the crew to reassess the situation, ensure all personnel are accounted for, and maintain safety protocols. This directive is essential in managing risk and maintaining control during complex aviation operations. It emphasizes the importance of discipline and communication among aircrew, ensuring everyone acknowledges and responds to the command promptly to prevent mishaps.

In contrast, the other actions suggested in the available choices would not adhere to the safety and control requirements signaled by 'Knock it off.' Resuming normal operations, changing altitude, or executing a tactical retreat could potentially exacerbate the situation if there are ongoing safety concerns. Hence, the only appropriate response is to cease all combat maneuvers as directed by the command.