Question

during the period of the era, atmospheric o2 level was almost equal to the present level, and multicellular life forms were abundant and diverse in the sea (but nowhere to be found, yet, on land).
cretaceous; mesozoic
quaternary; cenozoic
permian; paleozoic
cambrian; paleozoic
carboniferous; paleozoic

Explanation:

During the Carboniferous - Paleozoic era, atmospheric oxygen levels were high, similar to present - day levels, and there was abundant and diverse multicellular life in the sea with limited land life at that stage.

Answer:

Carboniferous; Paleozoic