* Why upsert?
* The upsert statement inserts records that don't exist and updates existing records based on the external ID (Order Number__c in this case).
* Salesforce automatically recognizes and uses the external ID field for matching.
* Why Not Other Options?
* A and B: merge is for merging records, not for upserting.
* C: Adding the field name after upsert is unnecessary, as Salesforce detects the external ID automatically.
* Upsert Documentation: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode
/langCon_apex_dml_upsert.htm
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