The correct answer is A. Two Master-detail relationships on a new custom object.

This is a classic many-to-many relationship requirement: one Contact can relate to many Solar Projects, and one Solar Project can relate to many Contacts. Salesforce recommends using a junction object for many-to-many relationships. A junction object is a custom object with two master-detail relationships.

The new custom junction object would sit between Contact and Solar Project, with one master-detail relationship to Contact and one master-detail relationship to Solar Project.

C could technically connect records using lookups, but the standard Salesforce data-modeling answer for a true many-to-many relationship is a junction object with two master-detail relationships.

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