Showing passed questions again

The variable route is one way to go. Another way would be to use an object on or off-screen and change its state based on whether the answer to the question was correct or incorrect.
When revisiting, you either show or skip the question based on the state of the item. 
...and while that seems simple enough, it isn't quite because you will have to preserve the final state of the slide, but have some trigger to determine whether to skip it or not. This probably also means having layer involved that is reset on revisiting to initial state.

The point is, it can be done without variables, but it might be just as painful to set up. Reach out to me on LinkedIn if you want to discuss it further, we can set up a call.

Hi Owen,

Thank you so much for your answer.

Your solution is based on correct/incorrect answer, but I would like to base my test on answered/unanswered question.

Learners take the test with 4 question banks (possible to put the questions in simple scenes however) and they must have the possibility to press a "Skip question" button for example.

The Skip question button is easy to put in place.

But how may I show the unanswered questions up when they have been skipped?

I want to use slide master, so mistakes would be harder to make for my colleagues when modifying the tests.

Got it. You could do this with JavaScript and by storing an Array as a text variable in Storline. 
When the user skips a question, you would execute JavaScript to pull the Array and the question number from Storyline, convert the array from text to numeric, append the array by adding the question number to the end, and sending it back to Storyline as an updated value in the Storyline Array variable.

Next, on review of skipped questions, you will do the opposite. On trigger (probably a next button click) execute Javascript to pull the Array from Storyline, isolate the first value in the array and separate it into its own variable (removing it from the array), the sending the updated array and question number back to Storyline. You can use the question number retrieved from JavaScript to guide the navigation to the question that should be reviewed.

Hi Owen,

That sounds great! Do you know where I could find the javascript code for that? Because I know nothing to Javascript...

Connect with me on LinkedIn and I'll walk you through it.  We can schedule a call. I just don't want to drop a meeting link here in the community forum.

Hi Owen,

Thanks a lot, I've sent you a Linkedin message. :)

I did a mock-up of using JavaScript to create a "skip" bank of questions.

You can see it in action here: LINK