* A: An External ID field is designed to store an ID from another external system, which can be used to match Salesforce records with data from an external source during data imports or integrations.
* D: External ID fields are automatically indexed, which can significantly improve the performance of SOQL queries when filtering based on the External ID.
Why not other options?
* B: External IDs cannot be formula fields. Only text, number, email, and auto-number fields can be set as External IDs.
* C: While External IDs are useful for integrations, they do not make external data visible within Salesforce Mobile.
References:
* External IDs Overview