An app builder just added a lookup field to Account from the Existing custom object,...

The lookup field is created on the custom object Box and points to Account. That makes Account the parent-side object and Box the related child object. Salesforce automatically creates a standard relationship report type in the format Parent Object with Child Object, so the report type is Accounts with Boxes. Salesforce Help confirms that standard report types can be generated automatically when reportable custom objects have lookup relationships.

Why others are incorrect:

A. Boxes with Accounts starts from the child object perspective.

C and D include “with or without,” which is normally configured through custom report type relationship options, not the automatically created standard relationship report type.

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