How should a data architect implement the storage of master key within salesforce?

The MDM solution provides de-duplication features, so it acts as the single source of truth.

How should a data architect implement the storage of master key within salesforce?
A . Store the master key in Heroku postgres and use Heroku connect for synchronization.
B . Create a custom object to store the master key with a lookup field to contact.
C . Create an external object to store the master key with a lookup field to contact.
D . Store the master key on the contact object as an external ID (Field for referential imports)

Answer: D