The correct answer is B. Lookup.

This is a one-to-many mentorship relationship: one experienced Realtor can mentor many new Realtors, but each new Realtor works with only one experienced Realtor. A lookup relationship is appropriate for connecting one record to another in this type of flexible parent-child model. Salesforce relationship documentation confirms that lookup relationships are used to relate objects without the strict ownership, sharing, and deletion behavior of master-detail relationships.

Master-detail is too restrictive because the new Realtor record should not necessarily inherit ownership and deletion behavior from the mentor. Many-to-many is incorrect because each new Realtor has only one mentor. Indirect lookup is used for external objects, not this internal Salesforce relationship.

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