Which safety feature in the pit serves to stop the escalator automatically?

The stop switch is a critical safety feature designed to halt the escalator's operation in emergencies or malfunction situations. When activated, it immediately cuts power to the escalator, bringing it to a stop and preventing further movement. This feature is crucial for ensuring the safety of passengers, as it allows for an instantaneous response to any incidents, such as an obstruction or malfunction that could pose a danger.

In contrast, other options such as the emergency alarm typically serve to alert personnel or users to the problem, rather than directly stopping the escalator. Safety nets may help prevent accidents from falls but do not affect the escalator's operation themselves. Floor detectors serve a different function related to sensing if something is blocking the escalator’s operation but are not the primary means to stop it. Therefore, the stop switch is the most direct and immediate method to ensure safety by halting the escalator when necessary.