Which of the following is essential for the certification of documents under international law?

The correct choice is significant because an apostille is specifically designed for certifying documents in a manner that is recognized internationally. This is crucial in the context of international law, as it ensures that a document from one country will be accepted as valid and authentic in another country that is a party to the Hague Convention Abolishing the Requirement of Legalisation for Foreign Public Documents.

The apostille serves to simplify the process by providing a standard format for verifying the origin and authenticity of public documents. This is particularly important in a globalized world, where individuals and businesses frequently require legal documents to be accepted across borders.

In contrast, notarization, while it may be part of the documentation process, does not carry the same international recognition unless it is further validated by an apostille or authentication from the appropriate governmental authority. Registration pertains more to the recording of documents and does not imply certification for international use. An affidavit, although a legitimate legal document, is a sworn statement and does not serve as a means of certifying documents for international law purposes. Thus, the apostille is essential for ensuring that documents are universally recognized across different legal jurisdictions.