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.

External IDs Overview

Comprehensive and Detailed Explanation From Exact Extract:

A (Correct):The renderAs="application/pdf" attribute in the tag instructs the Visualforce page to render the content as a PDF file when accessed.

Example:

Invoice

Incorrect options:

B:renderAs="pdf" is incorrect; the correct MIME type is application/pdf.

C/D:contentType controls headers but doesn't render the content as PDF.

B: Using partial DML statements (e.g.,Database.insert(list, false)) allows only valid records to be committed while identifying invalid ones, avoiding runtime errors.

D: TheDatabase.Savepointmethod ensures transactional control by allowing rollback to a specific savepoint if an error occurs.

Why not other options?

A: The@ReadOnlyannotation is for read-only operations and would not allow DML operations.

C: TheSystem.Limitclass helps monitor limits but does not directly control transaction behavior.

Best Practices for Apex Transactions