Correct option: A
See my explanation below:
- Formula field: it's a field, where the value is assigned from a formula you choose. It's read-only (not editable)
- Default value: it's an option in a field which allows (optionally) to define a default value for a field (es. Address field might have default value = street + door number). This option appears as default value for the field but CAN BE deleted when editing. So it's flexible.

The answer: The question wants to have a field essentially, that stores the value of the warranty with the flexibility of adding longer warranty as a way of increasing customer satisfaction, or BE ABLE TO EDIT THE FIELD. Because a formula field offers a STRICTER approach we would not be able to edit the value if needed. So the default value is the right option.

I hope this helps !

What is the exact difference between options A and B? and how does the extension of the warranty period relate to the question and answers?