Which material should have a flame spread rating of 25 or less when tested in accordance with UL 723?

The correct choice is insulation materials because the NFPA 96 standard emphasizes the importance of fire safety in commercial cooking operations, particularly in the context of materials used in kitchen environments. Insulation materials used in these settings must limit the spread of flames to reduce the risk of fire hazards.

A flame spread rating of 25 or less, as determined by tests according to UL 723, indicates that the material has a low propensity to ignite and allow flames to propagate quickly. This is crucial in kitchens where cooking operations can potentially generate sparks or heat that could ignite materials nearby.

In contrast, while the other options mention materials that have safety concerns, they do not necessarily need to meet the same stringent flame spread rating as insulation materials do under the NFPA 96 standard. For example, electrical insulation may not directly pertain to the fire behavior assessments outlined for cooking operations, and structural steel, while fire-resistant, does not typically carry a rating like insulation materials. Flammable liquids inherently pose fire risks and are managed differently under fire safety standards.