According to the exam guide, one of the objectives is to "describe best practices for implementing a single-org strategy in a B2C scenario"1. This implies that option D is the best practice for loading individual consumers as contacts in Salesforce. This approach avoids creating unnecessary accounts and reduces data duplication.
Option C is not correct because it creates one account per contact, which increases data volume and complexity. Options A and B are not correct because they do not leverage the standard contact object, which provides native functionality and integration with other Salesforce features.

Loading new records via the Insert operation and existing records via the Update operation will allow using the external ID field as a unique identifier and avoid any duplication or overwriting of records. This is faster and safer than deleting all existing records or using the Upsert operation, which might cause conflicts or errors .