A (n) ______ shall be used on the suction side of the hydronic pump.

The correct choice is to use an eccentric reducer with the flat side up on the suction side of a hydronic pump. This configuration helps in maintaining the proper flow of liquid and minimizes the risk of air pockets forming within the piping system.

In a hydronic system, the suction side should promote a smooth and uninterrupted flow to the pump, which operates more efficiently without the presence of trapped air. By positioning the flat side of the eccentric reducer upwards, any air that may enter the system is allowed to rise and escape rather than being trapped at the top of the fitting. This setup is particularly important because trapped air can lead to cavitation in the pump, causing inefficiencies and potential damage.

Other choices do not provide the optimal configuration for preventing air entrapment. For instance, using an eccentric reducer with the flat side down could potentially cause air to be trapped in the pipeline, negatively affecting the pump's performance. Meanwhile, concentric reducers would not be suitable in this case because they create a transition that doesn't allow for the same effective air management as the eccentric reducer does when properly oriented.