The useful clue is the business action being requested. The best answer is A. Flow. The selected option is not chosen because it sounds familiar; it is chosen because it gives Salesforce the right instruction at runtime.
Screen flows guide a user through prompts, collect input, branch through decisions, and update records as part of a controlled interaction.
The scenario describes a business outcome, not merely an administrative preference. If the feature is configured correctly, users get the intended result without depending on memory, spreadsheets, or manual policing. That is the professional standard for Platform App Builder work: place the rule in metadata, test it under the correct permissions, and keep the design supportable.
None of the other options gives the same administrative control. B (Apex) is only a partial match because Apex may solve advanced gaps, but it is not the first choice when a native declarative feature fully handles the requirement. C (Roll-up summary field) misses the mark because it addresses a neighboring feature area rather than the exact behavior requested. D (Formula field) is not enough: A formula is calculated display logic and cannot always store static values or perform record updates. This is the stronger design because it respects security, data quality, user experience, and lifecycle management at the same time.
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