The complete adjective clause is D.) THAT I HAVE WRITTEN.
An adjective clause fulfills 3 requirements:
1) it has a subject and a verb
2) it begins with a relative pronoun (who, whom, whose, that, or which) or a relative adverb (when, where, or why)
3) it functions as an adjective answering the questions what kind? which one? or How many?
That I have written.
Subject = I
Verb = have written
Relative pronoun = that
Function as adjective: which letter? letter that I have written.
