Final answer:
LP gas portable cylinders must be labeled and marked in accordance with US Department of Transportation regulations to ensure safe handling and transportation.
Step-by-step explanation:
LP gas portable cylinders must be labeled and marked in accordance with the United States Department of Transportation (DOT) hazardous materials regulations. This includes identifying the contents, the name and address of the person responsible for the cylinder, and any applicable warnings or safety information. The labeling and marking requirements ensure that the cylinders are handled and transported safely.