After you type the question and press
Enter, you are presented with the next number in the list. At this point, press the
Tab key to indent and the value for that line will change to "a". Continue through all the options for that question; each new line will be the next letter of the alphabet.
When you have finished a question (and its options), and you press "Enter", and you are presented with the next letter in the sequence, then press
Shift-Tab - the tab position will revert to the "number" position, and the value till change to the next number in the sequence.
If you'd like to put a blank line between questions, then you will need to press
Enter to create the new line though this stops the "Numbered list". To create a new question, select the "Numbered list" as usual - the value will be the number one, regardless of the previous number in the sequence. To advance the number to the next in the sequence, right click and select "Continue previous numbering".