A clean build puts the rule where Salesforce evaluates it. In this case, Choose A. There are already two master- detail relationships on the Invoice Line Item.; C. Invoice Line Item records exist without having the Invoice lookup field populated. Master-detail relationships create ownership, security, and lifecycle dependency between parent and child records. A junction object uses two master-detail relationships to represent a many- to-many model.
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 Data Modeling and Management, 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. B (Custom objects are unable to be on the detail side of a master-detail Relationship.) does not fit cleanly; Master-detail is powerful, but it is wrong when the records need independent ownership or when the child-parent direction is reversed. D (There is a roll-up summary field on the Invoice object.) misses the mark because it addresses a neighboring feature area rather than the exact behavior requested. The result is a cleaner app design because the behavior is handled by the feature Salesforce built for that purpose.
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