As first revealed in this discussion on the Quiz forum https://moodle.org/mod/forum/discuss.php?d=452115 , there is a problem in the way the Drag-and-drop markers manages right answers.

When the teacher ticks "Highlight drop zones which have not had the correct marker dropped on them", in the question settings,then the question / quiz displays not just the highlighted drop zones on attempt review, but also prints the correct answers inside EVEN IF of the "Show right answer" set to "No" in the quiz Review / Preview options. On the other hand, when the "Highlight drop zones" is disabled in the question, then the quiz's Review option to show "Right answers" is totally ignored. The two settings talk at cross purposes here.  

As pointed out by Dominique Bauer in the forum discussion, further complications arise when we use the interactive with multiple tries, where Hint 1 and Hint 2 options additionally set conditions for what to display, while the last (final) try is then "controlled" by the "Highlight drop zones", revealing too much when enabled, and hiding any correct answers when disabled: "whether 'Right answer' in the Review options is selected or not, the correct answer is never given if 'Highlight drop zones...' is not selected or always given if it is selected. What's missing is the condition corresponding to 'Whether correct' selected and 'Right answer' not selected."

Highly confusing is also the message: "Highlighted markers [sic !] are now shown with the correct placements. Click on the marker to highlight the allowed area."

• According to the option set in the question I expect to see the "highlight zones" and NOT the correct answers.
• Everything is plain to see at once, and no clicking is needed to "highlight" anything.

In summary, the "Show right answers" quiz review option should be made effective for this question type.