This scenario should be solved with standard metadata. Validation rules prevent bad data from being saved.
They are appropriate when the requirement is to enforce a condition at save time rather than merely warn, display, or report on the problem later. Use C. Create a validation rule that uses the PRIORVALUE function.
A clean Platform App Builder answer should satisfy the complete scenario, not merely one phrase in it. This item requires the app builder to choose the configuration that produces the requested behavior in normal Salesforce use. The selected answer fits that requirement because it works with the platform model instead of fighting it.
The alternatives are plausible only if the requirement is read too narrowly. A (Create a validation rule that uses the ISCHANGED function.) should be rejected because A validation rule blocks saves; it does not create records, route approvals, display components, or grant access. B (Create a formula default value for the custom fiel) would be fragile here because A formula is calculated display logic and cannot always store static values or perform record updates. D (Create a formula field that uses the PREVGROUPVAL function.) is only a partial match because A formula is calculated display logic and cannot always store static values or perform record updates. A complete implementation should also check page assignment, field access, record ownership, and mobile behavior where those factors apply.
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