Answer :
"which was to be delivered in unmarked five dollar bills" is the subordinate sentence, since "the note repeated the telephone..." is the main one.
Answer: The subordinate clause in the sentence is "which was to be delivered in unmarked five dollar bills." and it is used as an adjective clause.
Explanation: Like any clause that begins with a relative pronoun ("which" in this case), it cannot stand as a sentence on its own; it depends on the noun "ten thousand dollars" because it acts as a modifier of it. This function of modifying a noun applies only to adjectives, so this is an adjective clause.