Taxonomy: Show select options as a dropdown, not multiple
To get a taxonomy entity reference to show as a dropdown, there are two things you need to do.


Go to the "manage form display" (admin/structure/types/manage/MY_CONTENT_TYPE/form-display) for your content type and make sure the Widget for the term field is set to Select list.
Go to the "manage fields" for your content type and click "edit" for the entity reference for your terms.  Go to the "Field settings" tab and make sure the Allowed number of values is 1.  If this is set to 2 or more, Drupal will give you a multiple select, not a dropdown.