A dropdown question is defined by a few properties:
- The type of the question is either select_one (single choice) or select_multiple (multiple choice)
- The appearance will be either minimal or autocomplete
- The acceptable answers will be listed in the choices worksheet
Take a look at the following help topics:
Question Types: XLSForm essentials—Survey123 for ArcGIS | ArcGIS
Appearance: Appearance—Survey123 for ArcGIS | ArcGIS
You can also create a survey from the 'Appearance' sample form to see how a dropdown question is constructed.