Which of the following is a required pipeline marker?

The requirement for pipeline markers is that they must provide clear information about the contents of the pipeline, specifically indicating the type of materials being transported. This guideline is important for safety, as it informs first responders and the public about potential hazards in the event of a leak or emergency. The inclusion of information such as "warning with the type of materials like petroleum" ensures that anyone encountering the pipeline understands what substances may be present, reducing the risk of exposure and enabling appropriate response actions.

Markers that simply state a warning or caution without any details do not fulfill the safety requirements as they lack the necessary specificity that would inform individuals about the nature of the hazard. Similarly, while symbols can be useful, they must accompany clear descriptions of the materials for effective communication. Therefore, the correct answer emphasizes the need for both a warning and specific information about the type of material, aligning with safety regulations and protocols.