You can use an on key stroke action to set the variable of the question. So if you hit “a” on the keyboard, the action could be set to make the variable of the question be the value of the first answer. But you have to program the action for each answer AND program it uniquely on each page that has a question as the variables are different. But you can copy the page and then just reset the question variable and the actions. So you have a bit of a baseline.
It’s easiest if you make the actual answers of the questions be a,b, c, d and then use a text box for the answers that is NOT part of the question. That way you can just set the variable of the question to a,b,c,d.
Additionally, this will not show the selection on the page. So the radio button won’t be filled. But the variable WILL be filled and if the page was reloaded, the radio button would be selected. A unique way to present the question is to set the radio buttons to not initially visible and use a graphic like a check mark to display when the keystroke is hit. This would require two actions for every answer; one to set the variable and another to show the checkmark.
Hope this helps.