Creating dropdown problems in Open edX
Dropdowns are a good way of asking multiple choice questions a bit differently. In dropdown questions, students are pushed to search for the answer a bit more intuitively than if they were to simply recognize it from a list of options. Here's how they're put together in Open edX.
From edX Studio, navigate to the unit of your course where you would like to add a problem, then select the Problem button. 
You'll now be presented with a list of different common problem types. Select Dropdown from this list to insert a template version of this problem type.
Now that the template version of this problem is inserted, we need to edit it to fit the question you want to ask. Select Edit on the upper right of the component.
This will open the Simple Problem Editor. In this editor, it uses simple markup to denote different parts of the problem. If you're ever unsure, you can click the ? (question mark) button on the right hand side to open a cheat-sheet with the different markup you can use. The ones we need are as follows:
- >> << - The text enclosed by two chevrons is the question that is being answered
- [[ ]] - The square brackets denote a dropdown input to answer this question
- [[ incorrect, (correct), incorrect]] - Within the square brackets, the options available are listed, separated by commas. The correct answer to this question is enclosed by normal brackets/parentheses.
- [explanation] - Anything within the explanation tags is invisible until the learner selects Show Answer
Another neat feature about Open edX problems is that the default information it uses to populate the Simple Editor is an explanation of that problem type, so it's always right there with you. You should probably delete that introduction and replace it with your own text to contextualize the problem. Any text that just exists to give instructions, rather than asking a question doesn't need any markup, and should not be contained within the chevrons. And yes, you can use HTML in problems, if you know how to do so.
By selecting Settings in the top right, you can also change the problem type name that is something more specific than "Dropdown", as well as modifying a variety of other settings such as setting the maximum problem attempts, or configuring when the learner can give up and hit "Show answer".
When you finish editing your problem, be sure to hit Save so that your work isn't wasted!
That's all there is to it! You've now got yourself a functioning dropdown problem. Don't forget to Publish when you're ready for learners to see it!