'Tripped off' here means moved away with light, quick steps. After saying goodbye, she lightly walked or skipped off down the road — the phrasal verb 'tripped off' captures this sense of moving away briskly. 'Tripped over' means to stumble on something, 'tripped out' is informal for becoming disoriented (often drug-related slang), and 'tripped in' is not standard. Therefore option (c) is correct.
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