One Answer per Row
First enter your Question Text.
A matrix is essentially a grid of options. So what you will need to do is enter question Rows and question Columns.
The columns are the options that you can select for this question.
When you are creating a matrix of Choices with only one Answer per Row, you will only be able to select one of the options in the Columns.
When you are done click on Add Question.
This is how that question would look like.
Please note I can only select one column per Row.
Multiple Answers per Row
When you create a Multiple Answers per Row, you will be able to select multiple answers per Column.
First type in your Question Text.
Then the Question Options Row and your Question Options Columns.
This is how the Multiple Answers per Row will look like.
As you can see, I selected 2 answers for this example. One for the Left and the other for the RIght.