During migration from Blackboard some quiz/test questions came over as "multi-select" but are actually "multiple choice." That means that there is only 1 correct answer, but the quiz/test lets students select more than 1. When editing the question type, the drop down will not work.

In order to fix the question, it needs to be rebuilt. Scroll down to the bottom of the quiz/test and click on the caret next to the blue "Save" button and select "Save and Copy." Scroll back to the top of the quiz/test. The question type drop down now works. Click on it and select "Multiple Choice."

Next, click the bubble next to the correct answer. 

Finally, scroll down and click on the blue "Save" button. You can now delete the original "defective" question and drag the new one into its place.