Which API allows you to add a field group to an existing schema?

The Schema Registry API > Update Schema Attributes is the correct choice for adding a field group to an existing schema. This API is specifically designed to modify and enhance schemas in the Adobe Experience Platform. When you want to add new field groups, the Update Schema Attributes endpoint allows you to do so by defining additional fields or organizing them into new field groups within an already existing schema structure.

In contrast, options like the Catalog Service API > Modify Schema and the Identity Service API > Update Identity Schema serve different purposes. The Catalog Service is more geared towards managing the catalog of assets and schemas rather than altering schema attributes directly. Similarly, the Identity Service API focuses on identity-related data rather than schema modifications, making them unsuitable for the task of adding field groups.

The Field Group API > Add Field Group is not the right choice in this context because, while it sounds logical, it doesn't align with the specific implementation within the Schema Registry API. The Update Schema Attributes endpoint allows for such changes comprehensively as part of schema management, making it the most appropriate choice.