How can the consultant achieve this?

A local charity receives its income from recurring payments, The Recurring Donation object is used and contains a unique and manually entered reference number. This reference number should not be modified after creation. The finance department has requested that all child Opportunities also contain this unique reference number to make it easier to reconcile payments .

How can the consultant achieve this?
A . Create a custom text field on the Opportunity object and use NPSP Recurring Donation Custom Field Mappings
B . Create a text formula field on the Opportunity object and use Process Builder to update all child Opportunities
C . Create a custom field on the Opportunity object and deploy a trigger to update all child Opportunity records.
D . Create a custom text field on the Recurring Donation object and use NPSP Recurring Donation Custom Field Mappings

Answer: D