Explanation
The most likely cause of the custom fields in the Order Summary being empty in the export file is that the developer does not have access to the fields. To export data with Data Loader, the developer must have the View All Data permission and the appropriate field-level security settings for the fields they want to export. If the developer does not have access to the custom fields in the Order Summary object, they will not be able to export them. Option A is incorrect because a misspelling in one of the custom fields would not affect the export of the other two custom fields. Option B is incorrect because the Cart to Order action does support the mapping of custom fields from the Cart object to the Order Summary object. Option D is incorrect because the developer can export Order Summary records by using Data Loader or other solutions, not only by using Data Export Service. References: Export Unmanaged Order Summaries with Data Loader, Data Loader Guide, [Set Field-Level Security]