A gas or fluid line marked with the letters PHDAN is used to carry what type of substance?

The designation "PHDAN" indicates that the gas or fluid line is intended for the transportation of a hazardous substance. The letters in this marking are often used within industry standards to signify the presence of materials that pose a risk to health, safety, or the environment. When dealing with such lines, it is crucial to ensure proper handling and storage to minimize exposure to these hazardous materials, thus ensuring compliance with safety regulations and reducing potential liabilities.

The other options do not align with the typical standards associated with the "PHDAN" marking. Water, air, and fuel do not generally require such specific hazard identification unless they are being transported under conditions that could render them hazardous, in which case a different or additional marking would likely be used.