So that you know, prepare field will only work to populate choices in the admin when editing a post. When using get_field() or the_field() the field will return values selected but it will not return the correct label if the label is different than the value. Also, if you use get_field_object() the choices will not be populated in the returned field object.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users