what is the requirement for a metadata field to be used as a trigger field in profiles?

Correct answer: B

“The selected trigger field must satisfy the following criteria:
• It must be an option list metadata field.
• There is a built-in field called IdcProfile (which can neither be deleted nor changed, only the option list values can be edited). It is recommended to use IdcProfile as the Trigger field; but, this is optional. Any metadata field with an option list can be used as the trigger field.
• After the trigger field has been defined, it cannot be deleted from the system.
• It is, however, possible to “undefine” or disable the trigger field. The administrator can set the trigger field back to “none specified.” This disables all profiles.
• The trigger field may be changed. When it is changed, however, profiles may become invalid and it is the administrator’s responsibility to resolve the situation. However, user interface hints are provided concerning the profiles that are invalid.”