A dropdown matrix is very similar to a multiple choice matrix question except that it uses dropdown fields instead of a series of multiple choice variables. In other words, with a dropdown matrix question, you will be able to answer a given question about a number of variables.
How to Use:
In order to use a Dropdown Question, simply drag it from the list of questions (on the left side of your survey editor screen) and drop it onto the right side of your survey editor (where you see a live preview of the questions). Once the question has been added, you can view the question options on the left side
Watch this video tutorial on the drop down matrix:
More information here.