Use a Record-Triggered Flow optimized for Actions and Related Records.

Submitting a record for approval is an action in Flow. Salesforce provides a Submit for Approval action, and after-save record-triggered flows are used when the automation needs to perform actions or work with related records after Salesforce saves the triggering record.

Option A is wrong because Fast Field Updates is the before-save path, mainly for updating fields on the triggering record. It is not the right optimization for submitting an approval request. Option B still requires a custom button and manual user action. Option C is unrelated because this requirement is based on an Opportunity record change, not a platform event.

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