The "normal order for accessing a multiple choice quiz slide is (at least in my experience) as follows:
1st "Tab" reads the Slide Title if it is there
2nd Tab - reads the question stem
Using the Arrow Keys - You must now use the up and down arrow keys to access the answer choice radio buttons. When each radio button is selected, it SHOULD tell you the choice letter or number and read the text (not always the case)
Selecting the answer - The answer is selected by pressing the space bar when the proper radio button has been selected (up and down arrow keys). The radio button will be filled in when selected. DO NOT tab out of this selection until you are sure you have s chosen the answer you want - once the "Tab" button is selected, it can freeze your ability to change the answer
3rd Tab - jumps from the selected answer to the "Submit" button - choose "Enter" to to select "Submit".
You still haven't left the slide at this point, because the quiz slide requires you to click anywhere on the slide to continue after you select submit. Choose "Enter" again and the slide will advance to the next question.