Salesforce requires installation records to be directly associated with Opportunities, and sales reps need visibility of all installations tied to their Opportunities. A Master-Detail relationship is the correct solution because:

The Installation object (detail) inherits sharing and security from the Opportunity (master), ensuring sales reps see only their related installations.

Enables roll-up summaries on Opportunity if needed.

Salesforce documentation confirms that Master-Detail relationships provide parent-controlled ownership and access, making them ideal for enforcing related record visibility. (Salesforce Help)

B. Hierarchical is only available for User object.

C. Many-to-Many requires a junction object; not needed here.

D. Lookup allows optional, independent access; would not automatically enforce visibility.

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