The best actions to take to avoid creating duplicate leads when uploading a list of 400 leads are to utilize a Lead Matching Rule and corresponding Duplicate Rule to block newly created duplicate leads, and to upload the lead list using the import wizard and select a Matching type to prevent duplicate lead creation. The Lead Matching Rule defines the criteria for identifying duplicate leads, and the Duplicate Rule defines the actions to take when a duplicate is detected. The import wizard allows users to choose a Matching type based on the Lead Matching Rule and block or allow duplicates. Using Data Loader's update function or enabling Duplicate Matching in the Data Management section will not prevent duplicate lead creation.

Lookup is the type of relationship that would be set up to meet the specification of pairing experienced Realtors with new Realtors. According to the Salesforce documentation, "A lookup relationship creates a loose association between objects. The lookup field allows users to select a record from another object." Indirect lookup is used for junction objects on different platforms. Many-to-many is not a type of relationship, but a result of using junction objects. Master-detail is used for creating parent-child relationships.

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.