To perform DML operations that insert new records and update existing records in a list of Order__c records using the external ID field Order_Number__c, you should use:
Option D: upsert orders Order_Number__c;
Upsert Statement:
The upsert DML operation either inserts or updates records based on whether they already exist.
By specifying the external ID field (Order_Number__c), Salesforce will match existing records based on this field.
Syntax:
upsert orders Order_Number__c;
orders is the list of Order__c records.
Order_Number__c is the external ID field used for matching.
Reference:
"The upsert statement matches the sObjects with existing records by comparing values of one field. If you don't specify a field, the upsert statement uses the sObject's ID to match data."
- Apex Developer Guide: Upsert Statements
Why Other Options Are Incorrect:
Option A: merge orders;
The merge statement is used to merge up to three records of the same object type, combining them into one. It is not suitable for upserting multiple records.
Option B: merge orders Order_Number__c;
merge does not accept an external ID field as a parameter.
Option C: upsert orders;
While this will perform an upsert based on the record IDs, it will not use the external ID field Order_Number__c for matching, which may not find existing records without IDs.
Conclusion:
Using upsert orders Order_Number__c; correctly instructs Salesforce to insert or update records based on the external ID field.