Concept:A relative pronoun must match its function inside the relative clause, not in the speaker's inserted phrase like "I said".
Explanation:Look at the clause without the interrupter: "This is the man told me the interesting story."
It needs a subject pronoun: "who told me the interesting story."
Here, "the man" is doing the action "told", so "the man" is the subject of "told".
The words "I said" are only an inserted clause and do not make "the man" the object of any verb.
Therefore, the subject case "who" is correct, not "whom".
Answer:D. who