What type of circuits in dwelling units is designated to supply loads within that unit only?

The correct choice is branch circuits, as these are specifically designed to supply electrical loads within a dwelling unit. Branch circuits originate from the main electrical panel and distribute power to different areas of the home, such as lighting, outlets, and appliances within that unit.

Branch circuits are critically important to the safe and effective distribution of electricity in residential settings, ensuring that each area has the appropriate capacity and protection without overloading the system.

In contrast, other types of circuits serve different purposes. Main circuits refer to the principal circuits that deliver power into the dwelling and might not be limited to just one unit, as they can handle the overall distribution for multiple circuits. Sub-circuits is not a standard term defined in the NEC but might imply an informal categorization of circuits within a larger system, specifically referring to circuits derived from branch circuits. Service circuits typically point to those circuits that connect to the utility supply, thus serving a broader purpose than simply supplying loads within a single dwelling unit.