A Marketing Cloud admin runs the Contacts Counts report and sees there are currently 500,000...

The reason why the total contact count of 500,000 is less than the summed total of email subscribers, synchronized contacts, and mobile subscribers is D. There are overlapping contacts in each of the channels. This overlap occurs when the same contact is counted across different channels (e.g., a contact might be both an email subscriber and a mobile subscriber), thus leading to a lower total contact count when duplicates are not counted more than once.