@eddievelez & @prasunsen – actually the checkbox vs. radio box behavior is working as expected.
When you set the question as having more than one answer, checkboxes are used to allow users to select ALL the correct answers. By default, ALL correct answers must be selected for the response to be deemed correct.
If you would like to change this functionality so that any single correct answer from the set of correct answers qualifies as a correct response, you will need to check the box at the bottom of the question labeled, “Selecting any single correct answer is valid.” This will switch the inputs to radio buttons and force the user to only select a single answer.