Which of the following features is important for protecting a firefighter during thermal exposure?

High thermal insulation properties are crucial for protecting firefighters during thermal exposure because they help to prevent heat transfer from the environment to the firefighter's body. When facing extreme heat conditions such as those found in structure fires, the clothing must be capable of resisting and trapping heat to minimize the impact of high temperatures on the firefighter's body. Effective thermal insulation minimizes the risk of heat-related injuries, allowing firefighters to perform their duties more safely and efficiently.

While breathability, weight, and flexibility are also important factors in firefighter gear design, they are secondary to the essential need for adequate thermal protection. Breathability supports moisture management, weight affects maneuverability, and flexibility allows for ease of movement. However, without high thermal insulation properties, a firefighter is at an increased risk of exposure to heat stress and burns, which can be life-threatening in hazardous environments. Thus, high thermal insulation properties are the primary feature necessary for optimal protection in firefighting situations.