A production admin would not solve this by decoration. In this case, Choose B. Use field-level security on page layouts with record types to restrict editing fields. Lightning App Builder controls page composition, component visibility, activation, assignments, and specialized surfaces such as record pages, home pages, and utility bars.
The stem gives enough detail to reject surface-level fixes. Salesforce should be configured so the behavior is enforced or rendered by the feature designed for it. When the requirement touches User Interface, the builder should avoid crossing into a different layer unless there is a real platform limitation. The selected answer follows that discipline and creates the least surprising result for admins and users.
The remaining answers do not control the same Salesforce behavior. A (Use field-level security on page layouts to restrict editing fields.) would be fragile here because Page layouts affect presentation; they do not grant the underlying object or field capability by themselves. C (Use record types with field sets and restrict editing fields using field-level security.) is only a partial match because Permission controls are essential for access, but they do not perform calculations or process automation. D (Use field-level security to mark fields as read-only on the Sales profile.) misses the mark because Permission controls are essential for access, but they do not perform calculations or process automation. The result is a cleaner app design because the behavior is handled by the feature Salesforce built for that purpose.
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