According to code standards, what should device boxes be secured to when branch-circuit wiring is fished in existing hollow spaces?

The correct answer emphasizes the importance of securing device boxes to a finished surface. When branch-circuit wiring is fished in existing hollow spaces, the device boxes need to be attached to a surface that is complete and ready for use. This is critical for ensuring that the boxes are stable and can support the devices they will hold, such as switches or outlets. Securing boxes to a finished surface helps ensure safety and compliance with building codes.

It's important to note that having a secure attachment to finished surfaces allows for proper alignment and accessibility of devices after installation, contributing to a safer and more aesthetically pleasing outcome in electrical installations.