The correct answer is D) be sure no pressure exists in the line. Before disconnecting a joint in a pipeline, it is crucial to ensure that there is no pressure in the line. Disconnecting a pressurized pipeline can be extremely dangerous, as it can lead to the sudden release of the contents, potentially causing injury or damage. It is a safety requirement to verify that the line is depressurized before attempting any work on the joint. The other answer choices, while potentially relevant in other situations, are not directly related to the specific requirement of ensuring the pipeline is not under pressure before disconnection. For example, having a first aid kit on hand (A) and determining the size of the gasket (B) are good practices, but they do not address the primary safety concern of releasing pressure. Hanging a bucket under the joint (C) may be a practical step, but it does not address the underlying need to confirm the line is depressurized.