Which is required on a pipeline marker?

A pipeline marker serves vital functions in ensuring public safety and environmental protection. The inclusion of the pipeline owner's phone number is crucial as it provides a direct line of communication for individuals who may need to report issues, seek information, or require assistance related to the pipeline. In emergency situations, having immediate access to the owner’s contact information can facilitate rapid response and coordination with emergency services.

While the other options present potentially useful information, they do not match the primary purpose of pipeline markers. The size of the pipeline, depth underground, and the name of an emergency responder agency may not provide the immediate access to assistance that a phone number would offer in real-world scenarios. Therefore, ensuring that the marker includes the pipeline owner's contact information becomes essential for maintaining safety and readiness in the event of an incident involving the pipeline.