Drag&Drop quiz - oblige users to answer before continuing the course

Can you have a look at:

D&D Feedback messages - Captivate blog

You can use the same workflow used here to show the warning message, to hide/show an extra Next button.

You can trigger actions when a specific drag source object is dropped on a specific drag target object.  In order to gain access to these 'Object Actions' you first need to select the Drag Target object and then look for the Object Actions button in the Drag and Drop tab.

It can be tricky to set up but the usual approach is to set up User Variables for each of the Drag Objects and have the Object Action for that specific object assign its relevant User Variable to 1 if the object has been dropped on the correct target.  This will be done via the first decision block of a Conditional Advanced Action that is also unique for each drag object.  The second decision block in each of these Conditional Advanced Actions then checks to see if all of the variables have been set to 1.  If so, this would indicate that the user has successfully completed the interaction and then the Continue button would be shown.