Electrical equipment with ______ and low smoke and heat release properties can be installed within air-handling space (plenum).

The correct response highlights that electrical equipment must possess specific characteristics to be safely installed in air-handling spaces, also known as plenum spaces. These spaces are often utilized for air circulation in heating, ventilation, and air conditioning (HVAC) systems. Therefore, equipment installed in these areas must minimize the release of smoke and heat in the event of a fire to ensure safety and maintain air quality.

Electrical equipment that features a nonmetallic enclosure specifically listed for use in plenum spaces meets the stringent safety requirements needed for such installations. These listings ensure that the materials used in the construction of the enclosure will not emit harmful smoke or flame when exposed to high temperatures or fire conditions. Additionally, equipment contained within a metal enclosure is also acceptable since metals typically have low combustibility and good structural integrity, further reducing risks associated with fire.

The combination of these two types of enclosures contributes to reducing heat and smoke release, making options featuring either a metal enclosure or a suitable nonmetallic option appropriate for plenum installations. The inclusion of both allows for flexibility in selecting equipment while adhering to safety standards, which is why the choice encompassing both options is the correct answer.