The correct answers are A. Create owner-based sharing rules that give Read access to the master objects and C. Set Sharing Settings to Read Only on both Master-Detail relationship fields.
A junction object is normally created with two master-detail relationships to represent a many-to-many relationship. Salesforce documentation states that sharing access to a junction object record is determined by the user’s access to both associated master records and the Sharing Setting option on the master-detail relationship fields.
Because the master objects, Seminars and Attendees, have private organization-wide defaults, selected users need record access to those master records. Owner-based sharing rules can grant the selected group Read access to the master object records. Salesforce also documents that organization-wide defaults define baseline record access, and sharing rules are used to extend access when defaults are private or restricted.
The Read Only sharing setting on the master-detail relationship allows users with at least read access to the master records to work with related detail records, assuming they also have the required object permissions on the junction object.
B is incorrect because lookup filters restrict selectable related records; they do not grant record access.
D is incorrect because junction object records in master-detail relationships inherit sharing from the master records. Sharing is not handled directly through an owner-based sharing rule on the junction object.
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