For backing up metadata, which API should be used?

Using the Metadata API for backing up metadata is appropriate because this API is specifically designed to handle metadata operations within Salesforce environments. The Metadata API allows you to retrieve, deploy, create, update, and delete customizations in Salesforce, which include components such as custom fields, page layouts, and Apex classes.

When backing up metadata, you need to ensure that you capture all custom configurations, which is precisely what the Metadata API facilitates. It enables you to extract a full package of metadata in the form of XML files, making it a reliable choice for creating backups of the Salesforce configuration.

Other APIs, while useful for various tasks, do not primarily focus on metadata. The REST API is mainly used for interacting with Salesforce data records, the Bulk API is optimized for handling large sets of records with the data operations and is less suited for metadata tasks, and the SOAP API, similar to REST, is focused on data operations and transactions rather than specifically on metadata management. Therefore, the nature and purpose of the Metadata API align perfectly with the requirements for backing up metadata effectively.