D. This is the flag designer of whom I have spoken
"Whom" is a relative pronoun, and in sentences, it is used instead of ‘who’ as the object of a verb or proposition. In this case, "whom I have spoken" it's being used as the object of the preposition "of" as it may answer the question "of whom, who or what?", without it, the proposition would not make sense, and the sentence would be incomplete.
