Which of the following would be classified as a non-transient, non-community water system (NTNCWS)?

A non-transient, non-community water system (NTNCWS) is defined as a water supply system that regularly serves at least 25 of the same people for at least six months of the year, but it is not a primary residential facility. This characteristic clearly applies to the scenario where a water supply serves a factory with at least 25 employees, as the factory provides consistent water service to a stable group of individuals on a regular basis throughout the year.

In contrast, a water supply serving a campground may experience fluctuations in population as visitors come and go, which does not meet the criteria for NTNCWS. Similarly, a water supply that serves a small city is classified as a community water system since it provides water to a residential population, while a water supply serving a highway rest area does not meet the requirement of serving a stable population of at least 25 people consistently throughout the year.

Therefore, the situation with the factory aligns perfectly with the definition of a non-transient, non-community water system, making it the correct choice.