A company has a custom object named Region. Each account in salesforce can only be related to one region at a time, but this relationship is optional. Which type of relantionship should a developer use to relate an account to a region

Master - detail relationship

Explanation:

Master-detail relationship is the type of relationship that appears just like parent-child relationship, Therefore, Master-detail relationship can be defined both between standard object or custom object. The detail object receives the security and sharing settings that's also given to the master object, automatically.