Convert the Direct into Indirect speech: I said to her, "I will have written letters"?

I told her that she will have written letters

I told her that she would have written letters

I told her that I will have written letters

I told her that I would have written letters

The correct answer is: I told her that I would have written letters.
The original sentence is in the Future Perfect tense, and when converted to indirect speech, it becomes the Future Perfect tense in the subordinate clause, with the verb "would have" indicating the hypothetical or uncertain nature of the action.