What should be provided from the discharge side of the pump to facilitate filling?

The correct choice is to provide a tank fill line from the discharge side of the pump to facilitate filling. This setup is essential in mobile water supply operations, as it allows for the efficient transfer of water from the pump to the storage tank. The tank fill line is specifically designed to transport water in a controlled manner, ensuring that the tank can be filled rapidly and securely without spillage or loss of pressure.

In the context of mobile water supply systems, the tank fill line plays a crucial role because it provides a dedicated path for water to flow into the tank, optimizing the filling process. This ensures that the system can operate efficiently, reducing downtime and improving overall effectiveness in water supply operations.

The other options do not serve the same purpose. A drain line is designed to remove water, which would not help in filling a tank. A vent line allows air to escape or enter a tank, preventing vacuum formation, but it does not assist in the water filling process itself. A siphon line is typically used for transferring liquids from one container to another through atmospheric pressure, and can be less reliable in controlled filling situations. Thus, the tank fill line is the optimal choice for facilitating water transfer during filling operations.