The right answer to this question is option A. O2 in the atmosphere.
The main reason why it's a true that organisms would grow larger than we have today with more oxygen available is pretty simple to understand, their respiratory systems were smaller than we have today, and even though it was a small one, it was enough because of the higher presence and concentration of O2 in the atmosphere.
Some studies were held nowadays to check this possibility, to see if the O2 was the real main reason for this, and the animals used in it really grew bigger than usual, so by that, they confirmed their hypothesis by testing it.
