According to NFPA 291, how should fire hydrants flowing less than 500 gallons per minute at 20 psi be marked?

Answer: C — Red

In accordance with NFPA 291, fire hydrants that flow less than 500 gallons per minute at a pressure of 20 psi are designated as having a poor flow performance, which can significantly impact firefighting operations. To provide clear and effective communication regarding their capabilities, these hydrants are marked with the color red. The use of red for this category of hydrant serves as a visual cue to firefighters that these hydrants may not be sufficient for combating larger fires or when substantial water supply is required. The red marking ensures that responding personnel can quickly assess the hydrant's limitations, allowing them to make informed decisions on water supply sources during emergency situations. This standardized color-coding is a crucial element in maintaining operational efficiency and safety in fire response scenarios.