The diplomat's ______ tone — formal to the point of coldness — made it clear that any further discussion would be a waste of time.
Correct: C
Supercilious means having or showing an attitude of superiority and disdain; it describes a cold, condescending formality that dismisses others. The diplomat's tone of 'formal to the point of coldness' and the implication that discussion would be futile both align with supercilious. Deferential means showing respectful submission, effusive means expressing feelings in an unrestrained or excessive way, and magnanimous means very generous or forgiving — none of these match the cold, dismissive superiority described.