How can bottling the air be avoided?

Bottling the air refers to the situation where air becomes trapped in the brake lines, which can lead to ineffective brake operation. Leaving the angle cock open when cars are disconnected allows for proper airflow and prevents the build-up of pressure in the brake system. This practice ensures that when cars are uncoupled, the air can escape rather than being bottled up, which helps maintain the integrity of the brake system and allows for normal operation of the air brakes.

Each of the other choices suggests actions that could inadvertently lead to air being trapped. For instance, closing all angle cocks would isolate the brake lines, preventing the escape of air when disconnection occurs. Disconnecting all brake lines might create a significant safety hazard and does not specifically address the issue of air bottling. Ensuring the brakes are applied prior to disconnection could be useful for controlling the train but may not directly influence the prevention of air bottling in the brake system. Therefore, keeping the angle cock open during disconnection is the most prudent action to prevent bottling of the air.