An app builder wants to update a field on the parent record When a child...

The correct answer is A. Record-triggered flow.

The app builder needs automation to run when a child record is deleted and then update a field on the related parent record. Salesforce states that a record-triggered flow can automatically perform actions when a record is created, updated, or deleted, without user interaction.

Autolaunched flow can perform logic, but it does not run automatically on record deletion unless invoked by another automation. Validation rule only prevents invalid data entry. Quick action requires user interaction and is not suitable for automatic updates after deletion.

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