Two Master-detail relationships on a new custom object is how an app builder should configure the data model to create a relationship between Contact and Solar Project objects where each Contact can be related to multiple Solar Project objects, and each Solar Project can have multiple Contacts associated with it. This is an example of a many-to-many relationship that requires a junction object with two master-detail relationships. One Master-detail relationship on Contact and one Master-detail relationship on Solar Project, two Lookup relationships on a new custom object, and one Lookup relationship on Contact and one Lookup relationship on Solar Project are not valid or correct ways to create a many-to-many relationship.

Explanation
The app builder should recommend configuring Campaign Hierarchies and Campaign statistics to understand return on investment for the latest advertising buy. Campaign Hierarchies allow users to organize campaigns into parent-child relationships based on common goals or themes, such as product lines or marketing channels.
Campaign statistics allow users to track and measure key metrics for campaigns and campaign hierarchies, such as budgeted cost, actual cost, expected revenue, number of leads, number of contacts, etc. By using these features, users can calculate return on investment for each campaign and campaign hierarchy by comparing actual cost and expected revenue. Utilizing Account Hierarchies and Roll-Up Summary fields is not a valid recommendation, as it does not relate to campaigns or advertising buys. Account Hierarchies allow users to organize accounts into parent-child relationships based on business structure or affiliation, such as subsidiaries or partners. Roll-Up Summary fields allow users to aggregate numeric values from child records related to a parent record by a master-detail relationship. Running an opportunities pipeline report is not a valid recommendation, as it does not provide information about campaigns or advertising buys. An opportunities pipeline report shows the stages and amounts of opportunities in the sales process, such as prospecting, qualification, negotiation, etc. Changing to a public security model is not a valid recommendation, as it does not affect the ability to track and measure campaigns or advertising buys. A public security model is a type of organization-wide default that determines the baseline level of access that users have to records they do not own. A public security model grants all users read or read/write access to all records, regardless of ownership.