C. Update the PrimaryId__c field definition to mark it as an External Id:

Marking PrimaryId__c as anExternal Idallows Salesforce to use this field to match records during the data import process, streamlining the process of mapping CSV rows to existing Candidate__c records.

External Id fields are indexed and can be used in tools likeData Loader,Data Import Wizard, orAPI-based upsertsto ensure that records are matched correctly based on the value of the external ID field.

In this case, since PrimaryId__c is unique, marking it as an external ID eliminates the need for custom triggers or flows.

Why this is the best solution?

This technique leverages built-in Salesforce capabilities (no need for custom code or automation).

It ensures accurate record matching and is the standard approach for data enrichment tasks involving unique identifiers.

Why not the other options?

A. Upload the CSV into a custom object related to Candidate__c:

This approach unnecessarily introduces another object, adding complexity. The goal is to update existing Candidate__c records, not store the data in a separate object.

B. Create a before insert trigger to correctly map the records:

Using triggers for this purpose is overengineering. Salesforce already provides tools likeupsertfor this exact use case. Writing a trigger would require additional effort and could introduce unintended errors or performance issues.

D. Create a before save flow to correctly map the records:

While flows are powerful, they are not the optimal solution for this use case. Data mapping is better handled using an external ID during import to ensure proper matching and updates.