Answer
One could test for signs of the reversion in part a by looking at features such as gene sequences. If gene sequences were more similar to proposed + relatives near each - taxon, that would indicate the multiple reversions had happened.
Work Step by Step
The key is to imagine what would show that the reversion had actually happened. There should be signs of being more similar to nearby - taxa (version a) rather than more similarity to the ancestral taxon (b).
