(1. This is the easier.
2. This is the easier one.
3. This is easier.
Which is correct?)
This is a test question from an English group. I think the question is invalid.
2 and 3 are OK.
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I see all three as correct.
! and 2 relate to a context in which you are considering only two 'things'.
3 sounds like you think some other 'thing' is easier than either of these two.