Concept:The relative pronoun must match its role inside the clause: subject or object.
Explanation:In this sentence,
I said is a parenthetical expression inserted into the relative clause.
If we remove it momentarily, the clause becomes:
who told me the interesting story.
Clearly,
who is the subject of the verb
told.
The man performs the action of telling, so he is the doer, not the receiver.
A subject-form pronoun requires
who, while an object-form pronoun would require
whom.
Option C (
whose) shows possession, and option D (
which) is used for things rather than people.
Therefore, the sentence is correctly completed with
who.
Answer:B. who