Which fossil organism in whale evolution do you think was the first to live mostly in water? explain your claim with evidence and reasoning?

The evolution of whales was a gradual process in which terrestrial animals have become more and more adapted to an aquatic environment.  

Ambulocetus was perhaps the first whale ancestor that live mostly in water. Evidence suggesting this are the sediments where its fossils where recovered. These were most probably an ancient estuary.  

This species had large feet but short limbs, perfectly adapted for swimming.  The back limbs were longer than the forelimbs suggesting that together with their powerful tail, they were used for propulsion, in a similar fashion to modern river otters.

This species is a perfect example of a transitional fossil that shows how terrestrial animals can evolve into aquatic animals.

a. decomposes use carbon from dead organisms as fuel for cellular respiration, returning carbon dioxide back into the atmosphere.

yes, because even though a theory has a  lot of evidence to support it, it is not always true