There is nothing in the question, or in the Quick Lesson, that indicates that the phrase " tôt" is incorrect. There is no context, so BOTH/EITHER answer(s) must be correct. Annoying.
Tôt and en avance mean two different things, both of which are translated as "early" in English and can lead to confusion.
Tôt = early in an absolute sense (like, e.g., 4am)En avance = early, meaning ahead of some schedule (e.g., the train arrived early)
There's been a discussion with examples in the discussion of the lesson further down, I believe.
Perhaps you should file a complaint, there could be an error/omission?
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