A production admin would not solve this by decoration. Roll-up summary fields calculate values from child detail records and store the result on the master. The relationship type and field type decide whether the calculation is available declaratively. The correct response is therefore B. This lookup relationship will need to be converted to a master-detail relationship before a roll-up summary field can be created.
The scenario is not asking for a feature list. It is asking which configuration produces the requested result with the fewest moving parts. A Salesforce app builder should favor a standard, visible Setup configuration whenever it fully meets the business need. That approach makes the build easier to validate in sandbox and easier to explain during release review.
The remaining answers do not control the same Salesforce behavior. A (The roll-up summary can be achieved by creating a formula field on the `delay_` object.) would be fragile here because A formula is calculated display logic and cannot always store static values or perform record updates. C (A roll-up summary field will need to be created on the `delay_` object with a field filter that selects all related `Size_` records.) is only a partial match because it does not provide the same direct administrative control as the selected option. D (The roll-up summary can be achieved by creating a formula field on the `Size_` object.) misses the mark because A formula is calculated display logic and cannot always store static values or perform record updates. The result is a cleaner app design because the behavior is handled by the feature Salesforce built for that purpose.
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