The trick for using Drop Zones when making number line products is to create a series of Drop Zones, one at each possible answer. After you have several zones, wire up the draggable items to the correct answer drop zones only. If the students drag to the wrong drop zones, the answer will be marked as incorrect.
Do not mark any of the drop zones as correct or incorrect answers or your deck will complain that students have not found all the correct answers. Drop zone work by wiring up draggable answers to drop zones rather than by tagging answers as correct or incorrect.