Dropdown 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 problem types. Select "Checkboxes." You should now see something that looks like the image below. In order to edit your question. Click "Edit" on the upper right.
This is the Simple Editor where you edit simply. In order to change anything, all you have to do is type, but be mindful of how edX has the problems formatted, and try remain within those boundaries.
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.