Which relationship can be selected in a custom report type where Accounts is the primary object and Contacts is the related object?

This describes the relationship from the perspective of a Contact as the primary object to Account as the related object, not vice versa.

Contacts in Salesforce typically must have an Account, but this choice describes the relationship if Contact were the primary object.

In a custom report type, when Accounts are the primary object and Contacts are related, you can choose to show Accounts with or without Contacts. The option 'Each account may or may NOT have related contacts' (represented by 'A' records with or without 'B' records) accurately describes this 'outer join' type relationship, ensuring that all Accounts are included, even if they have no associated Contacts.

It is incorrect to state that an account must NOT have related contacts, as contacts are intrinsically linked to accounts in Salesforce's data model.