This question is really testing ownership of the feature. Choose C. Two master-detail relationship on a new custom object. In Salesforce terms, this is a relationship modeling issue. 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 selected configuration is the professional answer because it is native, declarative, and placed at the right scope. If the requirement later expands, another administrator can inspect the same metadata area and understand how the behavior is being produced. That is far better than a solution that depends on hidden assumptions or user training.
The other options fall short for clear platform reasons. A (One Lookup relationship on Contact and our Lookup relationship on Solar Project) is not enough: A lookup relationship is more loosely coupled and does not provide every master-detail behavior, especially native roll-up behavior. B (The Lookup relationship on a new custom object) handles a different concern; A lookup relationship is more loosely coupled and does not provide every master-detail behavior, especially native roll-up behavior. D (Our Master-About relationship on Contact and our Master-About relationship on Solar Project) should be rejected because Master-detail is powerful, but it is wrong when the records need independent ownership or when the child-parent direction is reversed. Before release, test the configuration with the affected profile and realistic records, not only with a system administrator account.
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