Create a master-detail relationship field on Applicant__c to Application__c is what the app builder should recommend to meet the requirements of allowing the same applicant to apply for multiple open positions using a single application. This will create a many-to-many relationship between Applicant__c and Open_Position__c using Application__c as a junction object. Create a master-detail relationship on Open_Position__c to Application__c, create a master-detail relationship field on Application__c to Open_Position__c, and create a lookup relationship field on Applicant__c to Application__c are not valid or correct ways to create a many-to-many relationship.