question on a layer

You can't convert the individual layer into a freeform question because when a slide is converted to a freeform question it builds the quizzing logic/structure to include layers.

In your example, I'd recommend using individual slides if you want to track the questions. The end user wouldn't know that they're jumping to slides and not layers.

Create a main slide that allows them to access the questions. Trigger to a question slide rather than a layer. Convert those slides to freeform. Trigger back to the main slide to select other questions.