Which type of box would not typically be classified as an outlet according to NEC definitions?

The correct option is a single gang box. In the context of the National Electrical Code (NEC), an outlet is specifically defined as a point in the wiring system at which current can be taken to supply utilization equipment.

A single gang box, while it serves as a support structure for mounting devices like switches and receptacles, does not, by itself, provide a means to take electrical current for devices. It is simply a housing to organize and protect electrical connections, rather than a point where power is available to connect to equipment.

Conversely, doorbells and light fixtures are classified as outlets because they represent points in the circuit where electricity is utilized—doorbells provide power to ring a chime, and light fixtures are designed to house light bulbs, thereby allowing for illumination. Receptacles are explicitly designed to allow for plug-in connections of electrical devices, which further underscores their classification as outlets.