Option B: Use an Assignment Flow element.
Correct Answer.
Using a record-triggered flow, you can automatically assign new cases based on the case origin.
Flows provide flexibility to set field values, assign records, and automate complex logic without code.
While an Apex trigger can perform the assignment, using declarative tools like Flow is recommended for maintainability and ease of use.
Validation rules enforce data integrity by preventing invalid data from being saved.
They do not assign records or automate processes.
Option D: Use a formula field on the Case object.
Incorrect.
Formula fields are read-only and cannot change record ownership or assign records.
They calculate values dynamically based on other fields.
Conclusion:
The best solution is to use an Assignment Flow element, which is Option B.
Reference:
Record-Triggered Flows
Automatically Assign Cases
Option C: Use a trigger on the Case object.
Possible but Not Best Practice.
Apex Triggers
Option A: Use a validation rule on the Case object.
Incorrect.

Explanation/Reference: