Maybe you could do the same as outlined in the image shared below:
In the survey tab of your xlsform:
In the choices tab of your xlsform:
Data entry screen as seen in Enketo: when nothing is selected
Data entry screen as seen in Enketo: when Cat A is selected
Data entry screen as seen in Enketo: when Cat B is selected
Data entry screen as seen in Enketo: when New Cat is selected
Reference xlsform:
Auto Select.xls (29.5 KB)