What feature must outdoor storage areas for flammable and combustible liquids have?

Outdoor storage areas for flammable and combustible liquids are required to be graded to divert possible spills to prevent hazardous situations. This feature is essential because it helps manage potential leaks or spills, directing any liquids away from areas where they could cause harm or create fire hazards. Grading the land properly can help contain any spills, minimizing their spread and reducing the risk of ignition. This proactive approach is crucial for ensuring safety in environments where flammable materials are stored, as it helps to protect not only the immediate area but also surrounding structures and personnel.

The other options do not address the critical safety concern of managing spills effectively in outdoor environments, which is why grading is the correct answer.