When attempting to change a master-detail relationship to a lookup relationship, Salesforce imposes specific restrictions to maintain data integrity:
C . The Account record includes Parks roll-up summary fields. Master-detail relationships allow roll-up summary fields to aggregate data from child records onto the parent record. If any roll-up summary fields are present on the master object (Account in this case) that depend on the detail records (Parks), the relationship cannot be changed to lookup until these roll-up summary fields are removed.
To resolve this, follow these steps:
Identify and delete all roll-up summary fields on the Account object that reference the Park object.
Convert the master-detail relationship to a lookup relationship.
Recreate any necessary roll-ups using declarative tools like Process Builder or Flow if needed, as lookups do not support native roll-up summaries.
For more guidance, review Salesforce's documentation on Changing Field Types.