Organisms of the early archean had to live: with an abundance of plant life without oxygen in the atmosphere completely on land and not in water with toxically high levels of oxygen

The answer is most likely: 'with an abundance of plant life without oxygen in the atmosphere '. The atmosphere contained almost no free oxygen during the Archean. It does not appear as if this epoch had an abundance of plant life, but rather mats of single-celled photosynthesising cyanobacteria. So I am not completely sure that either of the given choices are correct.

No they’ve not done that

The answer is b, as with an inference you make a statement with no evidence