Which type of device would most likely be installed within a ceiling box?

A lighting fixture is the most appropriate choice for installation within a ceiling box. Ceiling boxes are specifically designed to support and provide secure mounting for lighting fixtures, including ceiling lights, pendant lights, and chandeliers. These boxes are constructed to handle the weight and load requirements associated with lighting installations, ensuring safety and compliance with electrical codes.

In contrast, power outlets are typically installed at wall level and not in ceilings unless specifically designed for a particular application, such as a ceiling-mounted outlet for certain types of fixtures. Circuit breakers are part of the electrical panel and are not installed in ceiling boxes; they serve a different function related to circuit protection. Thermostats are usually mounted on walls and are not suited for installation in ceiling boxes, as their design and function require accessibility and visibility to control temperature effectively.

The specific design and intended purpose of ceiling boxes make them ideal for lighting installations, aligning with the requirements established in the National Electrical Code.