The correct answer is A. Flow.

The requirement says the Opportunity should store the Account Billing State value, but the value must remain static after the Opportunity is created. A record-triggered Flow is the recommended declarative automation because it can run when an Opportunity is created and populate a custom field with the related Account’s Billing State. Salesforce Trailhead describes record-triggered flows as automation that can run when a record is created or updated, and can update the triggering record or related records depending on configuration.

D. Formula field is incorrect because formula fields are automatically calculated and read-only. A formula would continue to reflect the current Account Billing State, so it would not preserve the original value after creation.

B. Apex is not the recommended first choice because Flow handles this requirement declaratively.

C. Roll-up summary field is incorrect because roll-ups aggregate child records; they do not copy a parent Account field value onto an Opportunity.