Should be C. Upsert will update using external id field and you do not have to have separate strategy for insert and update.

D
Upsert is less efficient than insert and update operation.

Should be C. Using the Upsert operation allows you to insert new records and update existing records in a single operation based on the external ID field. This approach avoids the need to delete existing records or perform separate insert and update operations. It provides a more efficient and streamlined process for loading both new and existing contact records.