A Combo Box question is similar to a checkbox question (otherwise known as a multiple choice, multiple answer question) except that it uses a select box rather than checkboxes in order to ask the same question. The advantage of using this question type rather than a checkbox question is that it displays a long lists of choices in a better format (i.e. appears shorter – with a scroll bar). In other words, this question type is similar to a dropdown question in that it allows you to display a long list of choices in a smaller screen area but also allows you to select multiple answers rather than just one.
How to Use:
In order to use a Combo Box Question, first drag a checkbox question from the left menu. Once the Options menu for the question appears, go to Display. The first line is Appearance, click on the accompanying drop down menu and the first thing you should see is the Combo Box option. The picture above is what is should look like!
Below is a video on checkbox questions, where you may find some relevant information: