The platform gives a native tool for this situation. Use B. The lookup relationship will need to be converted to a master-detail relationship before a roll-up summary field can be created. 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 selected answer is stronger because it meets both the functional requirement and the maintainability requirement. In a Salesforce implementation, a solution that only works for one user, one page, or one data sample is not good enough. The correct feature has to behave consistently across the intended profiles, record types, apps, and devices that are part of the design.
The other choices would leave a gap in the implementation. A (The roll-up summary can be achieved by creating a formula field on the `Galaxy_` object.) misses the mark because A formula is calculated display logic and cannot always store static values or perform record updates. C (The roll-up summary can be achieved by creating a formula field on the `Star_` object.) handles a different concern; A formula is calculated display logic and cannot always store static values or perform record updates. D (A roll-up summary field will need to be created on the `Galaxy_` object with a field filter that reflects all related `Star_` records.) is less defensible because it is not the most maintainable declarative control for this design. In a real org, the builder should include negative testing as well, proving that the rule does not fire or display outside the intended conditions.
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