The new custom object ("Installation__c") must store installation details that are related to an Opportunity.
Since each installation should be tied directly to a single opportunity, and inherit ownership and visibility from that opportunity (as per "sales reps should have visibility"), a Master-Detail relationship is appropriate.
* Why B (Master-Detail) is correct:Exact Extract:"In a master-detail relationship, the detail record inherits the sharing and security settings of its master record." - Salesforce Help | Relationships OverviewBecause Opportunities already have ownership and sharing settings that determine sales reps' visibility, making Installation__c the detail record ensures that sales reps automatically see related installations.Exact Extract:"The master-detail relationship closely links objects so that the master record controls certain behaviors of the detail record, such as record ownership, sharing, and deletion." - Salesforce Help | Master-Detail Relationships Why B (Master-Detail) is correct:Exact Extract:"In a master-detail relationship, the detail record inherits the sharing and security settings of its master record." - Salesforce Help | Relationships OverviewBecause Opportunities already have ownership and sharing settings that determine sales reps' visibility, making Installation__c the detail record ensures that sales reps automatically see related installations.Exact Extract:"The master-detail relationship closely links objects so that the master record controls certain behaviors of the detail record, such as record ownership, sharing, and deletion." - Salesforce Help | Master-Detail Relationships
* Why not A (Many to Many):Many-to-many relationships are only used when multiple records on both sides need to relate (e.g., Opportunities linked to multiple Installations and vice versa). That is not described here.
* Why not C (Hierarchical):Hierarchical relationships are only available on the User object.
* Why not D (Lookup):Lookup relationships do not automatically grant visibility or inherit ownership, so sales reps might not see all related installations.
References (Salesforce Platform App Builder documentation / Study Guide topics):
* Salesforce Help | Relationships Overview
* Salesforce Help | Master-Detail Relationships
* Salesforce Platform App Builder Exam Guide | Data Modeling and Management