Based on the exact options provided, the correct answer is A. Create a master-detail relationship field on Open Position to Application.

The requirement says one applicant must be able to apply for multiple open positions using a single application. In Salesforce, the relationship field is created on the detail/child object. Therefore, to allow one Application__c record to relate to multiple Open_Position__c records, the master-detail field would be created on Open_Position__c and point to Application__c . Salesforce Trailhead confirms that in a master-detail relationship, the relationship field is created on the detail object.

Salesforce also uses master-detail relationships and junction objects to model many-to-many relationships.

Important note: In a fully normalized recruiting data model, a separate junction object between Application and Open Position is usually cleaner. But from the provided answer choices, A is the only option that supports one Application being connected to multiple Open Positions.

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