28. According to the Chemical Data Guide, which extinguishing agent should be used on a petrolatum fire in an open area?

Foam is the appropriate extinguishing agent for a petrolatum fire in an open area because it blankets the fuel, excludes oxygen, and prevents re-ignition across a wide surface. Petrolatum burns as a surface fire, and foam effectively suppresses vapors and provides sustained coverage in open areas, unlike CO2 and dry chemical which are better suited for smaller, contained fires.