A smoke or heat detector may be recessed in the mounting surface if it is:

The correct answer highlights a crucial safety aspect regarding the installation of smoke or heat detectors. When a detector is listed and tested for recessed mounting, it indicates that it meets specific safety and performance standards established by relevant authorities. This approval ensures that the detector functions effectively even when installed in an alternative position, such as a recessed area.

Mounting a smoke or heat detector inappropriately can lead to diminished performance and an increased risk of failure during emergencies. Therefore, always verifying that the product is officially recognized for such an application is essential.

The other options, while they may pertain to factors important for installation, do not directly address the regulatory requirements or safety standards that justify the recessed installation of a detector. Only a device that has been tested and approved for such use can guarantee optimal performance in those conditions.