This is called cascading select
. You could learn more on the same from our support article Adding Cascading Select Questions.
You could solve your case 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:
Data entry screen as seen in Enketo:
Reference xlsform:
Cascading Select.xlsx (12.0 KB)