Which of the following is a requirement for thermal and sound insulating materials in terms of fire safety?

The requirement for thermal and sound insulating materials in relation to fire safety is to have a maximum flame spread index of 25. This standard is essential because it indicates that the material is effective in resisting the spread of flames during a fire event. A low flame spread index means that the material will not contribute significantly to the fire's growth, thereby offering increased safety for occupants and allowing more time for evacuation.

In contrast, materials with a smoke-developed index over 450 would indicate a higher propensity to produce hazardous smoke during combustion, which is undesirable in fire safety scenarios. A minimum thickness of 3 inches or a requirement for non-combustible materials alone does not necessarily ensure the material’s fire-resistant properties are adequate for building standards. Therefore, focusing on the flame spread index is crucial for ensuring that the insulating materials meet fire safety requirements.