A safe suction system requires which of the following?

The correct choice indicates that a safe suction system is designed to ensure that liquid can be drawn from the tank without any backflow or restriction. In this context, having no check valve on the suction piping allows for an uninterrupted flow of product from the storage tank to the dispenser. A check valve could inadvertently trap vapor, which could disrupt the suction and hinder the flow of liquid, potentially leading to a loss of prime in the system.

In contrast, the other options involve aspects that might compromise the system's efficiency or safety. For instance, piping that operates at atmospheric pressure is not ideal for a suction system, as it must create a negative pressure to draw liquid effectively. Piping that slopes upward to the tank would also pose a problem because it could create a vacuum that prevents the liquid from flowing back down to the dispensing unit. Lastly, while having a pressure gauge at the dispenser can be useful in certain contexts, it is not a requirement for a safe suction system, as the gauge measures pressure rather than ensuring the safe operation of the suction system itself. These considerations highlight the critical nature of piping design in maintaining proper function of a safe suction system.